PTR OK, SPF, DKIM passed but email messages are marked as Spam by Google?

I have had similar few months ago. I would recommend you to send an email to [email protected] then you will get an email back containing all the results and scores. Then I think you will have a clear view of your the problem preventing your messages to get delivered to gmail inbox.

There are many reasons why a message might be flagged as spam. Having one domain send a message which is signed by a different domain should be more than enough to ensure it is flagged as spam. Quite simply, you need to get everything properly lined up. The signature needs to be the one for the sending domain, not the domain it is sending for. What you have right now is a message that looks like it's being spoofed.

The unfortunate nature of current anti-spam techniques is that they are imperfect and prone to false positives if the recipient has settings wound too tight. I don't have a specific answer for you but I think there would be value in trying a few troubleshooting steps:

• Is this happening for all messages to that domain? Or just some?
• If the sender re-sends the message later, does it go through? (Just trying to determine if block is due to flow-control or is message specific.)
• Can you send manual tests (for example from the command-line via telnet) and successfully send a message?
• If you send the same message body from a different location does the message still get quarantined/tagged as spam?
• Did this behavior start recently? Or has this been an on-going issue?
• Have your users ever been able to successfully send to this domain?

Hope this helps.