Sender domain must resolve

[Pablo Mart�nez Schroder]

I'm having some problems sending mail to a specific domain...

When qmail tries to send mail to [EMAIL PROTECTED] from our mail server
(212.49.139.237) and the sender is [EMAIL PROTECTED], bt.es mail
server says "Sender domain must resolve".

This don't happen when the mail server sends mails to others domain, so
I really do not have a clue of what can be happening. I think it's
related with DNS, but I don't know what can I be doing wrong, or what it
supossed to be the right configuration.

Thanks in advance!
-- 
Pablo Mart�nez Schroder
Departamento de Administracion de Sistemas