I get Relaying Denied when I POP into server

[YoloIts]

I have a Red Hat Linux server running Sendmail with Mailscanner.  It has a
private IP address behind my firewall.  I have a Windows domain controller
with no public IPs and no public domain name which is also behind my
firewall.  When I setup Outlook Express to POP into the Linux server I get
the following response from Outlook Express.



The message could not be sent because one of the recipients was rejected by
the server. The rejected e-mail address was 'smith at PACBELL.NET'. Subject
'test', Account: 'mail.ycoe.net', Server: 'mail.ycoe.net', Protocol: SMTP,
Server Response: '550 5.7.1 <smith at PACBELL.NET>... Relaying denied. IP name
lookup failed [172.28.16.254]', Port: 25, Secure(SSL): No, Server Error:
550, Error Number: 0x800CCC79



My Linux mail server and the Windows domain controller are both running DNS
and I have them set up to pull zone transfers but I still get the message
"IP name lookup failed"  What am I doing wrong or what do I need to do?



Thanks