people alerted me that they got an email bounce saying:
Final-Recipient: rfc822; andreas@interdubs.com
Original-Recipient: rfc822; andreas@andreaswacker.com
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; can't create user output file
it turned out that my local mail file that I keep as a backup was bigger than 1000 Megabyte. Seems to be that postfix (or whichever program delivers the mail locally to /var/spool/mail) does not like to write to files that are bigger than that number. Scary the file grew to that size within one year.