I have an email delivery issue. I run a small web-based business for teachers. Recently, we switched to Amazon SES (through the SMTP interface, from a dedicated IP) for our email verification and password reset emails. We've got DKIM, SPF, and DMARC sorted according to mxtoolbox, and we've delivered thousands of emails with the new system without a problem.
However, emails to one school district, which operates their email through Microsoft 365, are consistently not being delivered. I've spoken with the very helpful IT folks there, and they say that an email trace at their end shows no sign of the emails we sent. Whitelisting on their servers has no effect.
I found a thread here that indicates that the problem may be with Microsoft rejecting the email. Following the link there, though, I am unable to validate our IP since it belongs to Amazon. Microsoft has been unhelpful, noting only that they "do not see anything offhand for the IP that would be preventing your mail from reaching our customers".
The only error message I'm getting on the bounces is "An error occurred while trying to deliver the mail to the following recipients", with no details that I can see. I'd be happy to provide headers on bounced or successful emails if that'd be helpful. I'm new to running an email server and am running out of ideas.