Are you sure that the messages are not being delivered to your Gmail account but were delivered to your Fastmail account? If you send test messages from your Gmail account to your Fastmail forwarding address, they will not appear in your Gmail Inbox. You must look in
All Mail in your Gmail account to see these messages, which are tagged as from
me. Do you also deliver them to your Fastmail account so you can see them there?
I sent various test messages to Fastmail aliases, one which used SRS and another which did not use SRS. All of these were delivered to my Gmail Inbox, even when I sent with From set to
something @ example.org, a standard test address which has SPF set to
v=spf1 -all (which rejects all messages from that domain with hardfail).
It appears to me that Gmail accepts messages which fail SPF with either softfail or hardfail as long as DKIM passes. I can't easily generate a test message with DKIM failing, so I'm not sure what happens if DKIM fails.
So my guess is one of these is happening:
- The message is hiding in your Gmail account (but not in Inbox) or is being filtered in some manner.
- The message isn't arriving at the proper Fastmail alias which targets your Gmail account.
- DKIM (or some reputation issue other than SPF) is causing the message to be rejected.
Bill