Some correspondents have kolab_user@kolab_server instead of From address in their replies

Hi all,

the problem sketched out above is bugging me and some of my e-mail contacts 
since quite a while - I am wondering if anyone on this list can help to solve 
the issue:

Some e-mail contacts of mine have the following header information included 
when they reply to my emails:

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Johannes Ranke

This is misleading, as the email address [email protected] is not 
functional. Sometimes they try to send e-mail to that address which is 
annoying for both sides.

It seems that this only happens if the other side is using Outlook as their e-
mail client.

Is there anything I can do to avoid this from happening? 

Kind regards,

Johannes