Mail dont work

[ricardo]

Hello,
I got ispCP succesfully installed on my Ubuntu 8.04 LTS server
running perfect.
Everything is working fine.
I created a mail account info@domain.tld
and if i send a mail to any adress i recive on that e-mail
but if i send a e-mail back with my MSN e-mail i don't recive on my info@domain.tld.

If i send a e-mail from admin@domain.tld to info@domain.tld i recive it.
So i think its the server.

I don't know why but the postmaster of Hotmail sends me a file with the
following information:

Reporting-MTA: dns;bay0-omc2-s17.bay0.hotmail.com
Received-From-MTA: dns;BAY131-DS7
Arrival-Date: Thu, 17 Jul 2008 11:44:52 -0700

Final-Recipient: rfc822;info@ricardofarina.nl
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;550 "Unknown User"

Mmm... Can you guys or girls help me please?

Greets,
Ricardo Farina