[fetchmail]multidrop doesn't work

I have checked out fetchmail's multidrop mode to get mail (which was
sent to several alias addresses) from one mailbox. The .fetchmailrc
entry:

*****
user sympa(_at_)my(_dot_)domain with password mysecret to *
        options fetchall stripcr
        mda '/usr/sbin/sendmail -oem -f %F %T'
*****

Running fetchmail -v -v on that, you can see the problem:

*****
fetchmail: analyzing Received line:
Received: from mailout03.sul.t-online.de (mailout03.sul.t-online.com
[194.25.134.81])
        by post.webmailer.de (8.9.3/8.8.7) with ESMTP id KAA17784
        for <thorn-dev(_at_)my(_dot_)domain>; Fri, 25 May 2001 10:34:11 +0200
(MET
DST)
fetchmail: line rejected, post.webmailer.de is not an alias of the
mailserver
About to rewrite From: Claus Atzenbeck <claus(_at_)my(_dot_)domain>
Rewritten version is From: Claus Atzenbeck <claus(_at_)my(_dot_)domain>

About to rewrite To: <thorn-dev(_at_)my(_dot_)domain>
Rewritten version is To: <thorn-dev(_at_)my(_dot_)domain>

fetchmail: no local matches, forwarding to claus
fetchmail: about to deliver with: /usr/sbin/sendmail -oem -f
claus(_at_)my(_dot_)domain claus
*****

The last line shows that it is deliverd to the local user "claus", not
to "thorn-dev".

I guess the reason is this line "fetchmail: line rejected,
post.webmailer.de is not an alias of the mailserver". Probably because
the user name of my ISP's mailbox includes the domain. How can I parse
the username that only the name before the @ sign is used for
multidrop?

Thanks a lot for any help!
Claus