SES SMTP rejects bounce messages from Exim

[RFC5321] allows the reverse-path to be null (see Section 4.5.5 in [RFC5321]). In this case, there is no explicit sender mailbox, and such a message can be assumed to be a notification message from the mail system itself. When the reverse-path is null, this document defines the "MAIL FROM" identity to be the mailbox composed of the local-part "postmaster" and the "HELO" identity (which might or might not have been checked separately before). https://www.rfc-editor.org/rfc/rfc7208#section-2.4

So, a null return path is effectively the same as postmaster@HELO, but specifically null to prevent backscatter loops.

SES will rewrite the return path to UUID@MAILFROMdomain for all egressing outbound mail. If you have feedback forwarding configured, any NDRs that flow back to that return path address will be sent to the MAIL FROM address that you specified in the original request. https://docs.aws.amazon.com/ses/latest/dg/monitor-sending-activity-using-notifications-email.html

So, you will want to be sure that feedback does not result in another outbound NDR and cause a loop.