Exchange – recepients being rejected

Thanks for taking the time to help me out. I am not too IT savvy, although I was thrust into the position by my even less savvy exec’s, so you may need to dumb down your answer.

my users keep getting this error message:

The message could not be sent because one of the recepients was rejected by the server. The rejected email address was ‘…@…com’ …….
Protocol: SMTP, Server Response: ‘550 5.7.1 Unable to relay for ‘…@…com’, Port: 25, Secure(SSL): no, Server error: 550, Error number: 0x800CCC79

any ideas how i can fix this? frustrating because 90% of emails are getting kicked back.

Thanks again for your help!

Mike Gosselin