Mail issue: getting empty reply back from user@servername

[NNNils]

One of our users gets an empty reply back from username@servername whenever he sends out mail through our SMTP server to a user within the domain.

What can this be?

There are no autoresponders or so.

It happens on all the accounts within this domain.

[NNNils]

I just found out that the issue is a little different:

Whenever someone sends e-mail to *@userdomain.com he gets an empty reply back from userdoma@servername.com