sender domain is changed when sending email

[ibschreiber]

Hi,
we have a strange problem:
I have two internal domains and a public domain defined.
@intern.lokal
@sl.intern
@company.com

a user created in @sl.intern, sending an email to sl.intern or company.com works fine:
sender test@sl.intern is displayed in the inbox as test@sl.intern

but:
a user from @intern.lokal, sending an email to any of the domains gets changed:
sender test@intern.lokal is displayed as test@company.com which is wrong and answering to this mail returns an error as the test@company.com does not exist.

There are no aliases defined, the settings for @sl.intern and @intern.lokal are identical (as far as I can see).
The user settings are identical, too. No alias or wrong sender address in the settings.

Any ideas?
Thank you!
Michael