[Exim] SMTP error 501

Hello!

I am running exim 3.02 on a Linux-box. I have the following problem:
When exim gets a message for an unknown user it tries to send back an
error message. Sometimes the MTA of the sender refuses to accept the
error message with the following error:

1999-08-07 17:45:09 11CvoA-0003pF-00 ** someone@???
R=lookuphost T=smtp: SMTP error
from remote mailer after MAIL FROM: <> SIZE=12935: host
mail.somewhere.com [xxx.xxx.xxx.xxx]: 501 bogus
mail from
1999-08-07 17:45:09 11CvoA-0003pF-00 Frozen (delivery error message)

I know exim puts in a correct FROM-field in the mail-header (Mail
Delivery System <Mailer-Daemon@???>). Mails to this
address are delivered to "root", which works fine. I think the other MTA
tries to read the FROM in the envelop. Is there a way to satisfy the
other MTA?

Greetings and thanks, Jan Suchanek
--
Jan Suchanek, URG, University of Ulm, Germany
jan.suchanek@???
jan.suchanek@???

NVNC ID VIDES, NVNC NE VIDES
- Motto of the Unseen University, Ankh-Morpork -