SMTP Error 111 occurs when encountering issues while connecting with the remote SMTP server. This error may arise due to problems such as invalid sender domains or firewall restrictions. In certain mail server environments, like Amazon SES, the 111 error might be triggered by issues like invalid login credentials or blocked email sending ports by the recipient.

What's Causing This SMTP Error 111:

The SMTP 111 error can be attributed to:

1. Issues connecting with the remote SMTP server.
2. Invalid sender domains.
3. Firewall issues restricting the connection.
4. For certain mail servers (e.g., Amazon SES), the 111 error may result from invalid login credentials or blocked email sending ports.

How SMTP Error 111 Manifests in Different Environments:

SMTP Error 111 in phpmailer:

1. Case 1: "SMTP Error 111 - Issues connecting with the remote SMTP server in phpmailer."
2. Case 2: "SMTP Error 111 - Invalid sender domains in phpmailer."
3. Case 3: "SMTP Error 111 - Firewall issues restricting the connection in phpmailer."
4. Case 4: "SMTP Error 111 - 111 error due to invalid login credentials or blocked ports in phpmailer."

SMTP Error 111 in Jenkins:

1. Case 1: "SMTP Error 111 - Issues connecting with the remote SMTP server in Jenkins."
2. Case 2: "SMTP Error 111 - Invalid sender domains in Jenkins."
3. Case 3: "SMTP Error 111 - Firewall issues restricting the connection in Jenkins."
4. Case 4: "SMTP Error 111 - 111 error due to invalid login credentials or blocked ports in Jenkins."

How to Resolve SMTP Error 111 - Step-by-Step Solution:

To resolve SMTP Error 111, follow these steps:

1. Validate the recipient email address: Double-check the spelling and accuracy of the recipient's email address, especially the domain name.
2. Confirm SMTP authentication credentials: Ensure that you've provided the correct username and password for SMTP authentication.
3. Verify email sending port availability: Check that the email sending port is accessible on your end and is accepted on the recipient's end.
4. If the issue persists, troubleshoot the SMTP server for potential firewall issues by attempting to connect to the recipient SMTP server through your local computer. If unsuccessful, it may indicate a problem on the recipient's end, requiring you to contact them for further information.