[exim] Auto Answer

I am configuring may .forward file to have an auto answer message in
this way:
...
if personal then
mail to $reply_address
mail from myaddress@mydomain
subject "I am on holiday - Auto Answer"
file /var/spool/mail/example.com/user/.vacation.msg
endif
the auto answer message works, but it is sent also a mail from:
Debian-exim@mydomain
with no subject and no body
to the sender.
How can i avoid the mail from Debian-exim@mydomain to be sent ?
Regards,