Mails misclassified from vBulletin Forum Software

[Joerg Rohrer]

Hi

All of a sudden mails from a forum are getting blocked. The Mails are 
Plaintext. No attachment at all.
-------
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit
X-Priority: 3
X-Mailer: vBulletin Mail via PHP
-------

Amavis message is:

Banned name: text/plain,.exe
Content type: Banned
Internal reference code for the message is 16107-02/o37RcrbaIzwa


Any ideas were i could start debugging?

Thanks
Jörg