Faced the following issue
A user reported that emails were sent successfully, but Gmail recipients never received them.
There were no visible errors in Outlook, however mail flow logs showed the following rejection:
Message 11409168 to some@gmail.com bounced by destination server. Reason: 5.3.0 - Other mail system problem ('550', ["5.7.26 Unauthenticated email from gmail.com is not accepted due to domain's", '5.7.26 DMARC policy. Please contact the administrator of gmail.com domain', '5.7.26 if this was a legitimate mail. To learn about the DMARC initiative,', '5.7.26 go to', '5.7.26 https://support.google.com/mail/?p=DmarcRejection ffacd0b85a97d-4398f8d369fsi2713102f8f.155 - gsmtp'])
Cause
The email was sent:
- Through the corporate SMTP server
- But the From address was @gmail.com
Example:
From: Corporate User on behalf of some@gmail.com
Gmail checks:
- SPF for gmail.com → fails
- DKIM for gmail.com → missing
- DMARC for gmail.com = p=reject
Result: Gmail rejects the message as spoofed.
Why this happens
You cannot send as @gmail.com through a non-Gmail server.
Gmail enforces strict DMARC and blocks unauthorized use of its domain.
Fix
Choose one:
1. Use Gmail properly
Send through smtp.gmail.com with authentication.
2. Use corporate address
Send as user@company.com when using corporate SMTP.
3. Keep Gmail for replies only
From: user@company.com Reply-To: some@gmail.com
Infrastructure Engineer with hands-on experience in Windows Server, Active Directory, SCCM, Exchange, and Linux environments. Concentrated on resolving production issues and keeping systems stable and reliable.