There was an old thread on this topic, apparently now locked.
Mail sent by me to a friend with email address on a domain hosted by 1 & 1 is generating responses like
Final-recipient: rfc822; email@example.com
Diagnostic-Code: smtp; 554-kundenserver.de (mxeue112) Nemesis ESMTP Service not available
The mail is apparently delivered, though. A message from her to me received a similar response and was not delivered, although we originally assumed this was because it contained just a single URL which I wanted.
I can provide the full error message if required but I will need to spend a little time redacting the meaningful addresses and names.