Messages are being rejected by GMail with DMARC failures. Most of the messages are from well known senders such as Google, Microsoft, and the USDA. The senders' DKIM is in order and we have correct DMARC, SPF and DKIM settings with respect to our DNS. Gateways are in place per the documentation. GMail extra spam checks are turned off. Here's a rejection of the Sophos Central welcome email:
Confirm you have configured the gateways correctly
During step 2 of step #9 of configuring the GMail Gateway IPs it says:
9. Turn on:
The second step (Reject all mail not from gateway IPs) causes Sophos EMail to stop delivering messages and emails enter the Queuing... state. I have had to leave that setting off for any emails to get through.
Headers show all PASS for everything
Continued troubleshooting indicates that Smart Banners are breaking DKIM - the alteration of the message body causes a mismatch which Google decides must be spam. Creating a group with Smart Banners disabled and Quarantine for Impersonation Protection then adding my Google Workspace users to the new group seems to fix the issue.
Overnight logs indicate that this isn't the problem in it's entirety - DKIM is still indicated as broken in some messages at the GMail user, and a bit of testing indicates that it's due to URL rewrites altering the body of the message. Incidentally, reading headers of successful messages I found that turning off URL rewrites and refraining from adding headers guarantees the integrity of the message.
According to gmail article https://support.google.com/a/answer/60730?hl=en
"Set up the inbound gateway setting to identify the gateway’s IP address or range of addresses. Gmail doesn't do SPF authentication for messages sent from IP addresses in the Gateway IPs list. The inbound gateway should do DMARC checks. DMARC authentication is bypassed for incoming messages from listed hosts."
So confirm that the correct IP's are on the list and that the email comes from that IP
If these are correct then the question becomes why is gmail checking this?
I have double and triple-checked this and the only solution is to add each sender with a DKIM of "-all" who is rejected to the Gateways list. Google's support seems to not be able to see an issue with this, describing the behavior as normal. My solution, for now, is to disable Smart Banners for this tenant and rely on changing the Subject line.
Please ask google support to check your Gateway IP list in google workspace The email that fails DMARC does if come from one of those IP's? If so according to their documentation referenced above they should not be doing DMARC. I cannot see how this can be normal behavior if their document says it should not be doing this. It say inbound gateway should do DMARC checks, which is Sophos Central Email.