SMTP Error only with SOME MAil Clients but not with Outlook

[Metapro]

Maybe someone could help me with this crazy authentification problem:

With outlook and smtp authentification is sending via SMTP possible.

With programs, like tobit david or mozilla thunderbird the server stops login with an authentification error like: This Server needs Auth...

But this is activated!

The POP3 Access (with same password) is problem free.

I can't find the problem - why does outlook work with smtp and the other mail clients not?

In Plesk nothing is black listed and authentification is enabled.

Please could anyone help me to locate the error?

EDIT: Oh, this post should have been in plesk 7 for win area, maybe someone could move it....thx!