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Old 5 Jul 2019, 02:35 AM   #6
The "e" in e-mail
Join Date: May 2003
Location: mostly in Thailand
Posts: 2,853
Originally Posted by Mr David View Post
Regarding the 554 error code, it has four entries in mimecast's table of reasons. The only one I know it can't be is max email size exceeded.l.
The error shown in your initial post was the one I highlighted in my reply.

Originally Posted by Mr David View Post
My guess is that the heart of this problem is the reputation of the domains FM owns and operates. If any of them have ever been hijacked by spammers then FM subscribers using email addresses on domains of ill repute can become collateral damage in the struggle to control unsolicited email.
It is possible, I guess, though that is not my first idea. Anyway, you could check out that theory by creating an alias in one of the rarely used FastMail domains and see if the same issue exists when sending using the alias.

More often, spam checks are based on message content, and the IP address of the sending server. This is augmented with checks on things like DMARC, SPF and DKIM. Some servers, in particular, do not like DMARC settings that are too loose.
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