qmail problem with localhost.localdomain

[ergos]

Hi all,
sending mail from my Plesk 8.0.1 (CentOS 4.3) server I have the following problem.

This happens only sending to some domain:

554 <localhost.localdomain>: Helo command rejected: using my own HELO hostname

this mail comes from my mailer daemon.

I should change the localhost.localdomain?
If yes, to what?

Thanks in advance,
Filippo