We have a bunch of satellite offices that relay through our corporate mail gateway. The mail gateway handles things like scan-to-email from copiers, email from our internal fax gateway, etc...
Starting Monday morning-ish Google has been one of a few things apparently at random: * Accepting the message from our mail gateway and delivering it to Inbox or Spam * Accepting the message from our mail gateway and disappearing it (nothing in GSuite logs either) * Rejecting the message with "451 4.5.0 OK <some id> - gsmtp" After doing a bunch of testing and finding nothing wrong, I chatted with GSuite. The first tech kept insisting I sign in to gmail and get him mail headers from the messages that were magically disappearing. I finally convinced him to escalate me. The second tech arrived at the conclusion that it was the Message-Id header. Messages that were delivered had an externally-resolvable domain as part of the Message-Id header. Messages that were 'disappeared' had our internal domain (i.e. whatever.local) as part of the Message-Id. I renamed one of the copiers to use our external domain and suddenly it worked perfectly. The moment it was fixed, the tech said the problem was solved and closed the case. Maybe I'm getting old and forgetful, but I seem to recall the RFCs saying the Message-Id was nothing more than a tracking identifier and could be complete garbage. Can someone at Google shed some light on this? I don't think the correct fix for this is to spend days updating copiers and fax gateways at tens of locations...and I'm suspicious that the Message-Id domain is the root cause. -A
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