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srahmat
Staff
Staff
December 3, 2025

Troubleshooting Tip: How to troubleshoot email rejected by gmail due to message is not RFC 5322 compliant

  • December 3, 2025
  • 0 replies
  • 221 views
Description This article describes how to troubleshoot the issue where outbound email is rejected by Gmail.
Scope FortiMail.
Solution

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This is because there are multiple To headers sent from the sending server.

This can be verified by doing a packet capture in FortiMail. Take note, this can only be seen if the email between the sending server and the FortiMail is not encrypted by using SMTPS or STARTTLS.

The screen capture below is a sample of the packet capture in FortiMail for the email that has multiple headers. It shows the sending server sends multiple headers.

 

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If the email contains multiple recipients, the correct header should show like 'To: abc@domain-a.com, xyz@domain-b.com'. Below is an example of the To header that follows the RFC 5322 compliant.

 

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