Outlook email with Exchange Server 5.5

I was able to send/reply email to this below individual. Now I could not send to this person any more. However, I could email to the rest of the world. Why not this one?
The error states below.

jsnell@peregrinegroup.com’ on 6/4/02 10:02 AM Unable to deliver the message due to a communications failure
The MTS-ID of the original message is: c=US;a= ;p=Boston Housing A;l=BHAMAIL1-020604140107Z-24461

550 rejected: unknown local-part in sender

I check the DNS and all the config.
Any suggestions?

Help.

Thanks,

Galileoave@yahoo.com