It can happen that our Mail-Server does not accept a Mail from time to time:
Delay reason: SMTP error from remote mail server after end of data: host mail.bbmb.ch [62.12.131.35]: 451 Internal error - try again later - command 'data' failed unexpectedly> Delay reason: SMTP error from remote mail server after end of data: host mail.bbmb.ch [62.12.131.35]: 451 Internal error - try again later - command 'data' failed unexpectedly
This seems like a hint
http://www.gossamer-threads.com/lists/qmail/users/133629