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Old 19 Sep 2017, 04:58 AM   #3
BritTim
The "e" in e-mail
 
Join Date: May 2003
Location: mostly in Thailand
Posts: 2,518
I do not claim to be an expert in this area, and someone may turn up to correct my explanation, but I think this is roughly what is happening.

The first thing to understand is that there are two flavors of forwarding
  • A regular forward that create a new message sent from the account that originally received the message.
  • A redirect, where the message is sent from (in this case) FastMail's servers, but with the original headers indicating that the sender is (usually) not FastMail.
The regular forward should never be a problem. The redirect can be troublesome. In your case, Google sees a message coming from <some domain> using a server (FastMail) that is unexpected (depending on SPF, DKIM and DMARC settings for the sending domain). In most cases, since you have no control over the security settings for the sending domain, messages are subject to being discarded by Google. For any specific sending email addresses, you may be able to circumvent the issue by using a Gmail filter that instructs Google never to discard messages from that address. As a general matter, it is a known gotcha with using redirects.
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