The remote mail server's Trend Micro spam filter is blocking you. I mentioned this possibility in the first response that I posted, above.
If you follow the link that is returned in the 550 response:
http://www.mail-abuse.com/cgi-bin/lookup?ip_address=220.225.96.217You'll see that this IP address is on the MAPS DUL blackhole list.
Is this the address that your mail server reports? If so, that's the cause of your problem.
You'll need to work with your client to either get your mail server white-listed in their anti-spam software (if it provides that feature), or you'll have to work with MAPS to get your server's IP address removed from the DUL list:
http://www.mail-abuse.com/cgi-bin/nph-dul-remove?220.225.96.217
Another alternative is to use your ISP's mail server to relay mail for you, rather than sending mail directly from your in-house mail server. This is what many companies do when faced with similar issues.
- Gary Patterson