rewriting of "from:" part user domain

[Irek Rogala <jorgi@gdansk.supermedia.pl>]

I have noticed strange (for me) behavior of my system.
If some system send mail via my relay using e.g. from: <user@his.domain>
where "his.domain" is domain in my localnames my system rewrites "from"
part to something like this: <user@my.domain> where "my domain" is my
primary domain.

Example:

My system name is star.gdansk.supermedia.pl
User sends mail:

From: <costam@car.supermedia.pl>
To: <costam2@car.com>

My systems rewrites this mail to this form:

From: <costam@gdansk.supermedia.pl
To: <costam@car.com>

:((
Maybe somebody can tell me where to start searching to resolve this
problem???

User "costam" is not system user (not in /etc/passwd). 

-- 
Irek Rogala
SuperMedia O/Gdansk