Email management

The Ultimate Guide to Improving Email Deliverability for Product Teams in 2024

Nikita Navral
April 6, 2023
Learn about different ways to improve email deliverability in 2024 including checking email authentication records, throttling issues, client rendering, domain reputation management and more.
Important Notice - New Email Policies That Might Break Your Product Communication from 1st February 2024 - From Gmail

Email deliverability is a crucial aspect of modern-day communication and it's the difference between your message being heard or ignored. With the overwhelming amount of marketing emails flooding inboxes, it's easy for critical transactional emails like payment reminders, OTPs, and product updates to get lost in the mix. 

This not only affects the customer experience but also poses a significant risk to business success. Improving email deliverability is no longer a nice-to-have, but a must-have for businesses that rely on email as a primary mode of communication with their customers.

Almost 73% of organizations have encountered difficulties with email deliverability within the past 12 months potentially resulting in missed opportunities and revenue.

Matthew Ramirez, founder at Rephrasely reflects, ‘These days, things are much stricter. Companies can’t just send emails willy-nilly and expect them to be delivered. They need to be extremely careful about the content of their emails and the way in which they’re sent in order to avoid getting flagged as spam.’

But how do you secure a place in the customer’s primary inbox, while navigating the maze of government regulations, email server restrictions, and sophisticated spam filters? Let’s find out!

Guide to Improving Email Deliverability in 2024

To improve your email deliverability, it's important to identify the specific factors that may be causing issues. Our guide will help you pinpoint these problems and provide solutions to address them. By identifying the right issue, you can take targeted steps to improve your email deliverability and see better results.

Reputation Management

Did you know that a staggering 56% of emails are blocked by mailbox providers due to low domain reputation?

Your IP address and domain hold a reputation that is constantly monitored by email service providers (ESPs) and Internet Service Providers (ISPs) to determine the value and legitimacy of your emails. This reputation directly impacts the deliverability of your emails, as a low domain reputation can result in emails being blocked or sent to the recipient's spam folder. 

The inbox placement rate can vary greatly, with a range of 69% to 97% according to the 2020 Deliverability Benchmark report by Validity, and is influenced by the receiver server’s location as one of the IP reputation factors.

But that’s not the only factor that determines your IP and domain reputation, others also include

  • Sender Authentication: Ensuring your email-sending domain is verified and properly authenticated is critical to establish a positive reputation and avoid being marked as spam.
  • Email List Hygiene: Regularly removing inactive and invalid email addresses from your email list can help maintain a healthy reputation, as sending to these addresses can lead to high bounce and complaint rates.
  • Email Content and Frequency: The quality and relevance of your email content, as well as the frequency at which you send emails, play a big role in your reputation. Avoid sending irrelevant or excessive emails to maintain a positive reputation.
  • Complaint and Bounce Rates: Receiving a high number of spam complaints and email bounces can negatively impact your reputation.
  • Spam Traps and Blacklists: Email deliverability can be negatively impacted by sending to email addresses created by ISPs to catch spammers, known as spam traps. Staying off email blacklists by avoiding sending spam is also crucial for maintaining a good reputation.
  • Email Engagement: Encouraging opens, clicks, and responses can help maintain a positive reputation, while low engagement can negatively impact it.

Tools for monitoring IP and Domain Reputation

Knowing how to measure your IP and domain reputation is crucial, yet often overlooked. So, let's take a closer look at the tools available to help you understand your IP and domain reputation.

  • Google Postmaster: One of the most popular tools is Google Postmaster, which evaluates the number of emails sent and determines the IP and domain reputation based on the data. The overall score is classified into ‘Bad’, ‘Low’, ‘Medium/Fair’, and ‘High’, with the High score indicating an excellent track record of low spam rates.
  • Sender Score: Sender Score, offered by Return Path, assigns an IP address a score ranging from 0-100. This score is based on data from over 80 mailbox and message security providers globally.
  • BarracudaCentral: BarracudaCentral evaluates the IP and domain reputation of an email sender using metrics such as email volume, authentication, spam complaints, blacklists, and content. These metrics are combined to provide a comprehensive score, indicating the sender's reputation and the likelihood of their emails landing in the recipient's inbox.

With a clear understanding of your IP and domain reputation in hand, you can take action and enhance your score to ensure your emails reach the intended inbox with ease.

Email Authentication

Three main email authentication methods (SPF, DKIM, and DMARC) are used to verify email messages’ authenticity to ensure that the message is coming from a trusted source protecting against spam and phishing. 

  • SPF verifies the domain of an email sender, preventing spammers from using fake domains. 
  • DKIM adds a digital signature to the message header to verify authenticity, ensuring that the message has not been altered in transit. 
  • DMARC allows the sender to specify what to do with emails that fail SPF or DKIM authentication, protecting against phishing and other types of email fraud.

According to the 2019 Global DMARC Adoption report by 250ok, a mere 6.1% of domains have implemented the preferred "p=reject" policy, while only 20.3% of domains have published any level of DMARC policy.

Common Authentication issues impacting email deliverability:

Check these common authentication issues, how to detect them, and what steps to take to fix them for improved email deliverability.

Problem Problem Description Identifying Problem Solving Problem
Multiple SPF Records Having multiple SPF records can cause confusion because email servers might not be able to determine which SPF record to use, and therefore consider the email as malicious. You can check the SPF records for your domain using the command line tool, dig: (dig TXT <domain name>) You should consolidate all of your SPF records into a single record. The format of an SPF record should look something like this: v=spf1 mx a include:<domain name> -all
Long DKIM Signatures Some email servers have length restrictions on the header size and a long DKIM signature can cause the header to exceed the limit, resulting in delivery failures. You can also check the length of your DKIM signature using the command line tool, dig: dig TXT <selector>._domainkey.<domain name> You should shorten the length of your DKIM signature by reducing the number of headers that are signed and by using a more compact representation for your public key.
Incorrect DMARC Configuration If your DMARC configuration is incorrect, it can result in emails being marked as spam or even rejected by email servers. You can also check the DMARC record for your domain using the command line tool, dig: dig TXT _dmarc.<domain name> Ensure that your DMARC record is properly configured and that it aligns with your SPF and DKIM records. Publish a DMARC record with a policy of "quarantine" or "reject".
Incorrectly configured SPF records An SPF record must accurately list all the IP addresses that are authorized to send an email on behalf of a domain. You can also check the SPF records for your domain using the command line tool, dig: dig TXT <domain name> Ensure that your SPF record lists all of the IP addresses that are authorized to send emails on behalf of your domain. Also, your SPF record should be correctly formatted and it does not contain any syntax errors.
Missing or invalid DKIM signatures If a DKIM signature is missing or invalid, email clients may reject the message or mark it as spam. This can occur if the DKIM signature is not properly set up or if there is a problem with the domain's DNS settings. You can also check the DKIM signatures for your domain using the command line tool, dig: dig TXT <selector>._domainkey.<domain name> Ensure that your DKIM signature is properly set up and that it is valid. You should also make sure that there are no issues with your domain's DNS settings.

If you are looking for a step-by-step process to create the SPF, SKIM, and DMARC authentication check out this page: Authenticating Your Emails: A Developer's Guide to SPF, DKIM, and DMARC.

Email Throttling

Email throttling refers to the practice of limiting the amount of email that an email service provider (ESP) will send in a specified time frame to prevent server crashes or slowdowns. When an ESP throttles an email account server, it slows down the sending speed, limiting the number of emails that can be sent in a given time period. Check out these limits imposed by popular email providers:

  • Twilio- Sendgrid states that with the v3 Mail Send endpoint, the maximum number of recipients, including all recipients defined within the "to," "cc," and "bcc" parameters, must not exceed 1000 requests per second (RPS).
  • According to Sendinblue, the POST/v3/smtp/email and GET/v3/smtp/blockedContacts endpoints have a rate-limiting of 1000 requests per second (RPS), while all endpoints under /v3/smtp/{…} are limited to 300 requests per hour (RPH)
  • Mailchimp allows you to send up to 1,000 recipients via SMTP at one time.

Some of the most common causes of email throttling are:

  • Abuse Reports: Receiving a high number of abuse reports from recipients for sending unsolicited, non-double opt-in, or lacking a working unsubscribe link email can result in throttling.
  • Sending to invalid/old email addresses: Sending emails to outdated or inactive email addresses, especially if they are spam traps, can lead to throttling and IP blacklisting.
  • Sending poorly formatted content: Before sending, ensure that the content has been checked for spam. This is especially important for industries with high spam volumes such as Mortgage Lending/ Real Estate, Gaming, or Affiliate marketing.
  • Reaching Sending Limits: Each email service provider imposes a certain limit on the number of emails that can be sent in a day, week, or month. If a business surpasses the limit, the email service provider will start throttling the emails.

Common Response Codes during Email Throttling

Learn about common email response codes encountered during throttling.

SMTP Response Code Description Impact on Deliverability
450 Email throttled by the receiving server, will be retried later. Delays in email delivery
452 Recipient's mailbox is temporarily over quota Delays in email delivery
Email could not be delivered
421 Service not available, too many messages received Email could not be delivered
454 Temporary authentication failure Delays in email delivery
521 Domain does not accept mail Email could not be delivered

Avoid Email Throttling Using SuprSend’s fallback mechanisms: 

SuprSend offers a solution to the problem of email throttling by implementing intelligent fallback mechanisms. When a status report is of failure or not received from the vendor end, SuprSend routes the messaging via a different provider. Additionally, SuprSend is aware of the throttling limits of each provider and can route notifications through multiple vendors simultaneously, improving email deliverability. 

In the event of a failed delivery through one channel, such as email, SuprSend can automatically route the notification via another available channel, such as Slack or SMS, through its channel routing feature.

Spam Filter Evasion

Almost half (45.37%) of all emails sent in December 2021 were labeled as spam. 

Not only does being filtered out by ISPs' spam filters harm email deliverability, but also being marked as spam by the end-user impacts it more. Being marked as spam raises your spam score, decreases your domain reputation, and makes future emails more vulnerable to spam filters.

Simon Tokic, cofounder at Mind Methods states on this matter, ‘We always state to our clients that they need to comply with anti-spam regulations and the main one being is that the people on your list have either clearly opted in or engaged with your list otherwise you're at risk of being reported for SPAM and this can trigger emails going into the junk box moving forward.

Common Spam Triggers Impacting Email Deliverability

Discover the most common spam triggers and examples with this comprehensive table. Spam triggers are elements within an email that can cause email filters to mark it as spam.

Spam Triggers Explanation Examples
Overuse of keywords Using too many spam trigger keywords such as "free", "earn money fast", "limited time offer", etc. in the subject line or body of the email. Subject line: "Get Rich Quick with Our Free Offer!"
Body: "Sign up for our free trial and earn money fast!"
Inadequate Unsubscribe Link Not providing a clear and easily accessible unsubscribe link. No unsubscribe link, buried or hard to find the unsubscribe link, lack of clear instructions on how to unsubscribe
Unsolicited Messages Sending emails to people who have not explicitly opted-in to receive emails from you can cause email filters to flag it as spam. Sending emails to an old list of purchased or rented contacts, or emailing someone who has not given permission to receive emails
Poor Sending Reputation A history of sending low-quality or spammy emails can hurt your sending reputation and increase the likelihood of future emails being marked as spam. High bounce rate, high number of spam complaints, low open and click-through rates
Email format The format of the email, such as the use of HTML formatting, excessive images, or large attachments can also trigger spam filters. Excessive use of HTML formatting, large image or attachment files, use of colored fonts or background
Excessive capitalization Using excessive capitalization in the subject line or body of the email. Subject line: "GET YOUR FREE TRIAL NOW!"
Body: "Sign up for our limited-time offer and EARN MONEY FAST!"
Too many links Including too many links in the body of the email can trigger spam filters. Body: "Click here to learn more and sign up now! Click here to see our latest products! Click here to read more reviews!"

You can use SuprSend for preference management from customers, automatically reducing the spam rates due to right management. 


Email blacklists are databases of IP addresses and domains that have been flagged as sources of spam or other unwanted emails. These blacklists are used by email providers and filters to determine if an incoming message is likely to be spam or not. 

Technical names for these blacklists include Real-time Blackhole Lists (RBLs) or Domain Name System Blacklists (DNSBLs or DNS blacklists). If an IP address or domain appears on one of these blacklists, it may result in a decrease in email deliverability and an increase in the likelihood that legitimate emails will be sent to recipients' spam folders. 

You can check if your domain is blacklisted here: Email Blacklist Check - IP Blacklist Check - See if your server is blacklisted (

Common Reasons for Getting Blacklisted

Here is a table that provides examples of how domains and IP addresses can end up on blacklists, along with potential solutions:

Reason for Blacklisting Example Solution
Spam Complaints Sending bulk emails without consent from recipients Obtain explicit consent from recipients before sending emails, monitor complaint rates and take appropriate action
Malicious Content Sending emails with viruses, malware or phishing attempts Ensure that all emails are secure and free from harmful content
Invalid or Misconfigured Email Server Incorrectly configured MX records or SPF records Ensure that your email server is properly configured and all necessary records are in place
IP Address with a History of Spam Sharing IP addresses with other users who have a history of sending spam Use a dedicated IP address or a reputable email service provider
Bounce Rates High bounce rates due to incorrect email addresses or full inboxes Regularly clean and verify your email list to remove invalid or inactive email addresses

Email List Quality

Email lists experience natural attrition, with a yearly decrease of approximately 22.5 percent which underlines the importance of maintaining email list quality continuously.

An email list that is high in quality will ensure that your emails are delivered to the inbox of your subscribers, while a low-quality list may result in lower deliverability rates, spam filters, and a tarnished sender reputation.

There are third-party tools available that use algorithms and data analysis to identify invalid or low-quality email addresses and remove them from your list. Some popular examples include BriteVerify, ZeroBounce, and MailboxValidator. You can also check the logs on your email provider's dashboard for information on bounced emails.

Common Email List Hygiene Issues

Here are some negative impacts of poor email list quality on deliverability, and how to fix them:

  • Bounce Rates: If your email list contains invalid or inactive email address, the emails sent to these addresses will likely bounce back. A high bounce rate can indicate to email providers that you are sending emails to unengaged or invalid recipients, and your future emails may be filtered as spam. Identify and remove invalid email addresses that generate a "hard bounce" response when emails are sent to them.

SuprSend automatically removes the bounced emails from the list, saving you from the trouble.

  • Spam Complaints: If your list contains email addresses that have been marked as spam by previous recipients, your emails are more likely to be marked as spam by email providers. This can negatively impact your sender's reputation and lead to a decrease in deliverability rates. Use Email/ Complaint Feedback Loop systems that allow email providers to provide feedback to senders when a recipient marks an email as spam.
  • Low Open Rates: Poor list quality can result in low open rates, which is a signal to email providers that your subscribers are not engaged with your emails. Email providers may then filter your future emails as spam, reducing your deliverability rates. If you have a high number of inactive email subscribers, it can be a good idea to run a reengagement campaign to identify which subscribers are still interested in your content and which ones should be removed from your list.

Email Client Rendering

A market survey of 3 billion email opens by Litmus found that the Apple ecosystem, including MacOS and iOS, is the leading email client with a market share of 50.4%, followed by Gmail at 27.2% and Outlook at 7.8%.

Email rendering refers to the process of how an email appears to the recipient in their email client. It is well known that some email clients, such as Microsoft Outlook and Apple Mail, are known to have limited support for CSS and HTML compared to web-based email clients like Gmail and Yahoo! Mail.

Additionally, some email clients, like Outlook, have different versions which may handle rendering differently, making it important to test emails on multiple platforms and clients. If an email is not optimized for the specific email client, it may not display properly, causing the recipient to have a negative experience. Usually, every email provider gives an option to ‘Preview’ your email before sending it. 

Common Client Rendering Issues

Rendering Issue How to Identify/Solve
Images not displaying properly Check the file format and size of the images. Make sure they are optimized for email display. Use alt text to describe the images in case they do not display properly.
Misaligned or distorted layout Check the coding and make sure it is using tables and inline styles rather than floats. Use a CSS reset to eliminate the inconsistencies across different email clients.
Text formatting issues Use web-safe fonts and fallback fonts for different email clients. Avoid using advanced formatting options like text-shadow or text-stroke.
Link issues Test all the links to make sure they are working and pointing to the correct URL. Use absolute URLs instead of relative URLs.
Unresponsive design Use media queries to make your design responsive and adjust to different screen sizes. Test your design on different devices and email clients.

Engagement-Based Email Deliverability

According to a report by Campaign Monitor, in 2021, the average email open rate across all industries was 21.5% with an average click-through rate of 2.3%.

The more engaged your recipients are with your emails, the higher the likelihood that your emails will reach their inboxes and vice-versa. Various metrics, such as open rates, click-through rates, and conversion rates measure recipient engagement. These metrics give you an idea of how interested your recipients are in your emails, and how well they are performing.

Email engagement improves significantly when used for customer retention measures rather than customer acquisition. Leszek Dudkiewicz, head of marketing at PhotoAiD comments, ‘Customers defend against unwelcome emails but welcome information from companies they have purchased from. When buying, they can agree to receive emails or sign in to a newsletter list, which increases the chance of email deliverability. As a result, email marketing will serve more for user retention than user acquisition.’ That’s why ESPs recommend using only subscribed users for campaigns as they are more likely to engage with the content, which can positively impact your domain reputation. 

Common cause of less engagement

Here's a table that summarizes the causes of adverse engagement and how to identify and solve them:

Cause of Adverse Engagement How to Identify/Solve
Email Frequency Monitor email sending frequency and ensure that it's not too high. Conduct A/B tests to determine the optimal frequency for your audience.
Relevance Analyze recipient data to ensure that emails are being sent to the right people and that the content is relevant to them.
Timing Monitor open rates for emails sent at different times and adjust your email sending schedule accordingly.
Subject Line A/B test subject lines to determine which ones result in the highest open rates.
Personalization Personalize emails using recipient data, such as name and location, to increase engagement.
Unsubscribe Options Ensure that the unsubscribe option is easy to find and use, to reduce the chances of recipients marking emails as spam.

Bounced and Unsubscribed Emails

The average unsubscribe rate for most email lists ranges between 0.2% to 0.5% per email sent, which is considered normal and should not cause alarm.

Bounced and unsubscribed emails can have a significant impact on email deliverability, as they both serve as signals to email providers that a recipient is not interested in receiving emails from that sender. 

Email providers use various algorithms to determine a sender's reputation, and a high number of bounced or unsubscribed emails can be a red flag indicating that a sender's emails are unwanted or low-quality. As a result, email providers may start filtering the sender's emails into the recipient's spam folder, further reducing the chances of their emails being seen.

  • A hard bounce will result in the recipient's email address being marked as invalid, whereas a soft bounce may only result in temporary delivery failure. The ISPs use this information to update their internal databases and block future email deliveries to the same email address. As stated by SEOAnt's CMO Avin Wei, a common reason for this is “due to our app subscribers using fake emails to get a free trial.”
  • In the case of unsubscribes, the ISPs track unsubscribed email addresses and update their internal databases to reflect the change. This information is used to block future email deliveries to the same email address and also to update the sender's list.

An easy way to save yourself from this hook would be to collect only valid emails from active people and further segment them based on engagement. Lisa Dietrich, partner at Girokonto says, “We use double opt-in confirmation messages for subscription forms and utilize whitelisting techniques for our customer databases. Additionally, we regularly segment customer lists based on engagement levels and leverage transactional emails in order to optimize open rates and reduce unsubscribe rates.” 

Common Problem with Bounced and Unsubscribed Emails

Check out these commonly associated problems and how to identify them proactively.

Problem Identification
High number of bounced emails due to invalid email addresses Review email collection process
High number of unsubscribed emails Analyze email content and frequency.
Hard bounces due to blocked or nonexistent email addresses Use email verification tools to validate email addresses
Soft bounces due to full inbox or temporary server issues Monitor bounce rate and address the root cause.

Email Provider Impact on Deliverability

Each email provider has its own policies and technical infrastructure for managing email deliverability. Some providers prioritize high email deliverability, while others may emphasize security or affordability. These policies and technical infrastructure can affect the likelihood that your emails will reach your recipients' inboxes.

This is summarized well in this recent email deliverability report of providers by EmailToolTester.

Closing Remarks

The success of any email run depends on how well it resonates with its recipients, and that can only happen if the emails actually make it to their inboxes. However, with email providers using increasingly sophisticated algorithms to filter out unwanted or spammy emails, improving email deliverability has become more challenging than ever. The good news is that with the right strategies and tools, product teams can significantly increase the chances of their emails reaching their intended audience. 

By following the step-by-step guide outlined in this resource, product teams can optimize their email practices and stay ahead of the competition. Whether it's improving email authentication, optimizing email content, or monitoring email performance, every step counts in enhancing the deliverability of your emails. By taking the time to improve email deliverability, product teams can build stronger relationships with their customers, drive engagement, and ultimately achieve their business goals.

Written by:
Nikita Navral
Co-Founder, SuprSend

What’s a Rich Text element?

The rich text element allows you to create and format headings, paragraphs, blockquotes, images, and video all in one place instead of having to add and format them individually. Just double-click and easily create content.

Static and dynamic content editing

A rich text element can be used with static or dynamic content. For static content, just drop it into any page and begin editing. For dynamic content, add a rich text field to any collection and then connect a rich text element to that field in the settings panel. Voila!

How to customize formatting for each rich text

Headings, paragraphs, blockquotes, figures, images, and figure captions can all be styled after a class is added to the rich text element using the "When inside of" nested selector system.

Implement a powerful stack for your notifications

By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. View our Privacy Policy for more information.