another relay problem

[trec]

Hi,

I am on a RHEL3 box with Plesk 7.5 Reloaded.

Relay configuration checked on server requiere autentication for POP3 and unchecked for SMTP.

the problem is I have one client using Outlook 2003 that always get the 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1) error when he tries to send a message.

I am having to put it's IP on the white list in order to let him send e-mails. Other users using Outlook 2003 on the same server are not having these problems.

any clue?
any software that could be installed on the user computer that could make him look as a extrange user?

Thanks a lot.