I would suspect that the sendmail server is doing a DNS lookup of your host and
since it is a 192.168.x.x address it isn't in the DNS server and it has relaying
turned off (this is a good thing).  You might need to add a rule to sendmail to
allow it to relay mail from 192.168.x.x addresses, preferably only accepted on
the internal LAN interface.

> I have been having this problem for quite sometime.That is, whenever I
> reply to a mail from Outlook Express, I get the following error
> 
> The message could not be sent because one of the recipients was rejected by
> the server. The rejected e-mail address was 'plug-
> discuss@lists.plug.phoenix.az.us'. Subject 'Re: Adding a third IDE drive in
> Linux', Account: 'mail.esaravana.com linux', Server: 'mail.esaravana.com',
> Protocol: SMTP, Server Response: '550 5.7.1 <plug-
> discuss@lists.plug.phoenix.az.us>... Relaying denied. IP name lookup failed
> [192.168.0.83]', Port: 25, Secure(SSL): No, Server Error: 550, Error
> Number: 0x800CCC79
> 
> The server it is trying to connect to is a RedHat 7.1, sendmail and a Bind
> 9 DNS, which also gives all my home computers IP address through DHCP
> Protocal.
> 
> It's not a problem, when I use sylpheed from RedHat 7.2 client.
> 
> Any help is appreciated.
> 
> Sundar
> 
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