ok, here's what I found:
in /var/log/mail.info there were lines from sendmail & postfix/smtp for the
NEW_DISPLAYNAME@ourdomain.org
That seems wrong, because the emailaddess of the user has not been changed at all. (I changed just the display name)
The errors I noticed came from internal mail messages. I tried the telnet localhost:25 conversation to actually send a mail, and it came through just fine.
So, not yet sure where the
NEW_DISPLAYNAME@mydomain.org came from, and where postfix / sendmail have found that..?
What I did now: I have created an email alias for the user for
NEW_DISPLAYNAME@mydomain.org, to see if the errors disappear. If they do, we know that that was the problem.
I would of course still like to know where postfix/sendmail would find that (wrong) information...
Thanks for helping so far.
mj