I was also getting such an e-mail (for a forum where I am admin).
Rejected incoming e-mails do not show up under
admin/email/rejected. Could you add them there please? That would at least help with investigation.
Also the error message is confusing.
Recipient e-mail address
firstname.lastname@example.org equaled body of rejection e-mail notification by discourse
["email@example.com"]. Message ID was probably correct too (hard to check), just the from address was wrong.
No, it wouldn’t have fixed my issue too.
In my case I did send from a different e-mail address (different SMTP server). My e-mail program Thunderbird apparently has a bug to sometimes confuse the sender SMTP e-mail address when using multiple SMTP servers.