I'm still having the same issue. Here is what I've done
I have deleted all rows from tabke ost_email_account & reconfigured my email.
After I select Same As Remote Mailbox , I receive a 500 error.
I forgot to mention this yesterday, but I'm using a shared mailbox in M365 and deselect the option Strict Matching. According to this thread, this should work for both remote inbox & outgoing mails.
For me it's only working for inbox and only after some trial & error: upon first submitting the OAuth2 authentication (for remote inbox) it reported an error Email Mismatch: Expecting Authorization for support@company.com not myaccount@verbeke.com eventhough I had deselected the strict matching option. After some retries It did work.
When I configure OAuth2 separately for outgoing mails, I also receive this mismatch error, even after several retries.
It seems to me this option forstrict matching is not 100% bullit proof