Problem sending Mail through MS-Outlook

I’ve MS-Proxy 2.0 on Windows-NT 4.0(SP4) machine. Clients are using Windows 95 & 98 and they can’t send their mails but they can receive mails. An error generates…
“The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ‘aamir_riaz999@yahoo.com’. Subject ‘Test Mail’, Account: ‘FDM’, Server: ‘fsg6.fascom.com’, Protocol: SMTP, Server Response: ‘550 not local host yahoo.com, not a gateway’, Port: 25, Secure(SSL): No, Server Error:550, Error Number: 0x800CCC79”
If anyone can help me then reply.