SMTP Error 221 is an authentication error that can be attributed to one of the following four reasons:

1. Invalid username or password.
2. Incorrect SMTP server configuration.
3. Absence of SMTP authentication for the account.
4. Non-utilization of SSL or TLS.

What's Causing This SMTP Error 221:

The SMTP error 221 arises due to:

1. Invalid credentials: The provided username or password is incorrect.
2. Incorrect SMTP server: Attempting authentication with an incorrect SMTP server.
3. Lack of SMTP authentication: Failure to enable SMTP authentication for the specific account.
4. Non-use of SSL or TLS: Sending emails without employing SSL or TLS.

How SMTP Error 221 Manifests in Different Environments:

SMTP Error 221 in phpmailer:

• Case 1: "SMTP Error 221 - Invalid credentials in phpmailer."
• Case 2: "SMTP Error 221 - Incorrect SMTP server in phpmailer."
• Case 3: "SMTP Error 221 - Lack of SMTP authentication in phpmailer."
• Case 4: "SMTP Error 221 - Non-use of SSL or TLS in phpmailer."

SMTP Error 221 in Jenkins:

• Case 1: "SMTP Error 221 - Invalid credentials in Jenkins."
• Case 2: "SMTP Error 221 - Incorrect SMTP server in Jenkins."
• Case 3: "SMTP Error 221 - Lack of SMTP authentication in Jenkins."
• Case 4: "SMTP Error 221 - Non-use of SSL or TLS in Jenkins."

How to Resolve SMTP Error 221 - Step-by-Step Solution:

To resolve SMTP Error 221, follow these steps:

1. Check username and password: Ensure that the provided SMTP username and password are accurate.
2. Verify SMTP server host: Confirm the correctness of the SMTP server host. For instance, if using a Gmail account, use the Gmail SMTP host address (smtp.gmail.com).
3. Enable SMTP authentication: If not already enabled, activate SMTP authentication for the specific account. For Gmail accounts, enable "Less Secure Sign-In Technology."
4. Utilize SSL or TLS: If the error persists, configure the port according to your SMTP server documentation. Use port 465 for SSL and port 587 for TLS when sending emails via Google.

By addressing these aspects, you can successfully resolve SMTP Error 221 in both phpmailer and Jenkins environments.

‍