Problem with bouncing mail

Hello fetchmail people,

there's a problem I'm having with bouncing emails in fetchmail. I'm using
sendmail and fetchmail 5.6.5-1 on a Intel 686 with RedHat Linux. I hope
somebody can help me out.
The problem is that every time fetchmail retrieves an email from the email
server, and sendmail responds that the user is unknown, fetchmail removes
the email without sending a bounce message to the sender. No email is sent
to the postmaster either. This is what I get in the log file:

fetchmail: SMTP< 550 5.1.1 
<martinp(_at_)catv888(_dot_)extern(_dot_)kun(_dot_)nl>... User unknown
fetchmail: SMTP> RSET
fetchmail: SMTP< 250 2.0.0 Reset state
fetchmail: Saved error is still 550
fetchmail:  flushed
fetchmail: POP3> DELE 4
fetchmail: POP3< +OK Message 4 has been deleted.

Does anybody know how to solve this? 

Thanks,

Martin Bruggink