Exchange 2003: No Mails in and no mails out - but internal mailing works fine

Hi,

since weekend an old SBS2003 stopped delivering Mails. Nothing where done at this machine. Mailing from one Account to another within Exchange works. Those mails coming in the inbox of the recipient.
But if we send a mail to a mailadress outside Exchange it goes to sent-box. But isn´t delivered.
The queues all shows no error and 0 mails.
In event-log there are no errors.
Incoming Mails are successfully delivered to Exchange (we use an third-party-Pop2SMTP). But there the mail shows up at no place.
Like a big black (Mail-)hole...

Anybody out there still experienced with Exchange 2003 ?

Thanks for helping me !

loosain