What is “X-Mailer” or “User-Agent”?

X-Mailer and User-Agent headers identify the email client or software that composed the message. They're optional but commonly included.

Examples:

X-Mailer: Microsoft Outlook 16.0

User-Agent: Mozilla Thunderbird/78.0

X-Mailer: Mailchimp Mailer

These headers serve several purposes:

Troubleshooting: Knowing the sending software helps diagnose formatting issues, encoding problems, or client-specific bugs.

Analytics: ESPs and receivers can track which clients generate traffic.

Filtering signals: Some spam filters consider X-Mailer as one of many factors. Unusual or missing values aren't definitive but contribute to overall assessment.

Most email clients add these automatically. ESPs typically include their own identifier. The header is informational and can be spoofed, so it's not reliable for security purposes.

Some senders remove X-Mailer for privacy, which is perfectly acceptable. Its absence doesn't affect deliverability. However, inconsistency (sometimes present, sometimes not) could look odd.

It's the craftsman's mark on the vessel. Useful for identification, but not a guarantee of seaworthiness.