Interesting thanks. I can imagine that an email sending service wants to be in control of message-id's (would still be interesting to hear the actual rationale).
Perhaps SMTP could use an extension with a new (pipelined) command after DATA, so a client can retrieve the assigned message-id.
I wonder how common it is for message-id's to be overwritten. I must have run into this SES behaviour (along with other problems) when trying to use it as backup for outgoing mail for my mail server: My DKIM signatures include the message-id header, overwriting the header will invalidate them.
It is a bit painful for idempotency.