Host unreachable

I’m having trouble sending email to a user. I keep getting the error host unreachable. When he send me email I get it okay. This is the only eamil address that I’m having problems with. I have an Exchange server 5.5 service pack 4, WIN NT 4.0 service pack 6. Also test with my yahoo email account and was able to reach him from there, it’s only when I send from my companys domain that I get the host unreachable.

Help will be appreciated