Issues Sending Emails with AWS WorkMail – Any Solutions?

That 554 error from Mimecast usually indicates there's a problem with SPF, DKIM, or DMARC alignment on your side. You should make sure that the sending identity for Amazon SES is included in your SPF record, and verify that your DKIM settings are correctly aligned with your domain. Mimecast has increased their enforcement on unauthenticated emails, so this might be a case where the recipients have tightened their rules recently. Checking the email headers from a successful email could help you compare the authentication results.

It sounds like the recipient you're emailing uses Mimecast, and they're rejecting the emails likely because they're being flagged as spam. This can happen if Mimecast thinks the email doesn't meet their security criteria. You might want to check with the recipient if they can whitelist your email or if they’re seeing any issues on their end.

I don’t even use SES, just WorkMail. I don’t see any settings like that, but it’s a good idea to check the email headers from previous messages. I did check, and the DKIM settings were set up correctly. I found that I was missing the Custom MAIL FROM domain setting. After going through some improvements recommended by AWS, it looks like my emails are now sending!