I recently implemented a standardized email signature at the company I am working at. I am using remote images hosted on our website within the signature so that I can swap them out for fun themes for holidays and such.
I have run countless tests at mail-tester . com and all have come back positive. With the exception of our mail server being listed on three spam lists since our email is through Gmail and that is out of our control.
I have an issue where about two of our clients simply cannot receive email from us when sent with the signature. Without the signature, the mail delivers just fine but with the signature it just doesn't even get delivered.. not in inbox or spam folder. It's as if it was never sent.
I've tried everything from updating our website's SPF record and dkim signature to allow the Gmail server to send on our behalf (something that probably should have been setup long ago) and the problem still persists.
Can anybody think of any reason that our signatures would be blocked.. albeit by a very small percentage of our client's mail servers?
I did see the below question that was asked back in 2011 but my emails have passed every tester that I've found.