[CentOS] Wierd Networking Problem

Shawn Everett wrote:
> Hi Guys,
> 
> I'm 100% sure this is not a Linux issue but I want a second opinion...
> 
> I have a Linux server running CentOS.  It was been absolutely perfect 
> without any errors or problems.
> 
> Since Monday it has been unable to send email.  It is configured to act as 
> a smart host.  All email gets forwarded to smtp.isp.net.

Anything happen around that time?

Any firewall rules on that box?