Looks like the SMTP server you're relaying to is rejecting the sender address - maybe you have UBE controls or blacklists or something like that (I don't know MS Exchange so wouldn't know what to suggest here) - with Postfix you can have DNSRBLs, access maps, sender restrictions, etc to govern this.
Try
Code:
$ telnet the.windows.smtp.server 25
HELO xyz.com
MAIL FROM: <root@xyz.com>
RCPT TO: <someone@xyz.com>
DATA
Subject: Testing mail
Hello,
This is a test
.
QUIT
