can´t receive replies mails domain must exist

[ErnestoVasquez]

We have a server as server.xxx.com.mx using sendmail -v account@xxx.com.mx we don´t
have any problems, but when we reply mail to server we have error
When we try use the telnet in port 25 we have the next sequence:

telnet server.xxx.com.mx 25
Trying...
Connected to server.
Escape character is '^]'.
220 server.xxx.com.mx ESMTP Sendmail 8.9.3 (PHNE_29774)/8.9.3; Thu, 29 Jul 2004 2
0:58:06 -0500 (CDT)
HELO server2.xxx.com.mx
250 server.xxx.com.mx Hello root@server [xx.xx.xx.xx], pleased to meet you
MAIL FROM: account@xxx.com.mx
501 Sender domain must exist

The error said (501 Sender domain must exist) how fix this..?
Our sendmail.cf have a DS relay and Dj domain, and our /etc/hosts has
the IP addreess and hostnames with domain too..

we need configuring something else??

my plataform is HPUX 11i and my relay is MSExchage

!!Thank you everybody

[Steven E. Protter]

They server you are connecting to has no idea what to do with mail for xxx.com

When you relay mail to an Exchange server the Dj domain merely effects adressing.

Exchange is telling you the you need an account for the HP-UX server on the exchange server.

SEP