E-mail not sent, test e-mail returns an error

[Mornagest]

Hello,

I'm facing a annoying problem with vB4.2.2 PL6: I cannot send e-mail anymore from vBulletin.

There is no e-mail for notification, no e-mail sent from the contact form, no e-mail sent to users... well, no e-mail

I ran the e-mail diagnostic utility and this is its reply:

SMTP: localhost
sendmail_from: None
sendmail_path: -t -i

The fact is that I can send and receive e-mails from my server. The contact e-mail address works fine when I use it with SquirrelMail, BUT it doesn't receive contact e-mails sent from the forum form.

Would you have any idea?

Thank you in advance!