5.1.1 smtp;550 5.1.1 User unknown

Hi everyone,

I am having a problem with few email addresses that seem to get bounced back to my mail box. Here is the message i get when i try to send or reply to the message:

Your message did not reach some or all of the intended recipients.

      Subject:  flights to nc

      Sent:     6/16/2008 10:32 AM

The following recipient(s) could not be reached:

     btalyor@cpda.com on 6/16/2008 10:32 AM

            The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.

            <dmfdc1.DMF.COM #5.1.1 smtp;550 5.1.1 User unknown>

I am positively sure that i am using the correct email address, that is no typo....

Can anyone help please???

Thank you.