e-mail send problem

Hi,
I can receiped mail but can not send ( only this e-mail address).Before working fine (send and received ok).starting this monday only can received I try every things (not reply. not use from address books, not from contacts) still not work. Again only this e-mail site xxx.com.
error message:(from outlook not exchange server event ID).
The recipient name is not recognized
The MTS-ID of the original message is: c=US;a=;p=Organization;l=exchange-020627155532Z-1054
Our mail server was exchange 5.5 with sp3

This problem shoulb our site or other site ? Thank a lot.(Again only this site to can not send any other site working fine send and received ok.
Regards,
ayewinnie