Amazon SES rewrites the “Message-IDs” of emails sent through it. This breaks one of Discourse’s security features related to incoming email.
To make it work, you can disable the “find related post with key” site setting, but be aware that it does open up the potential for users to be impersonated by email.
I think it might actually be possible to actually fix the SES problem.
SES rewrites the message IDs, which is annoying, but it gives the client the new message id in the SMTP transaction. So if we can get hold of that at the point of sending the email then swapping some stuff around here might solve the problem:
None of the destination email addresses are recognized, or the Message-ID header in the email has been modified. Please make sure that you are sending to the correct email address provided by staff.