C#, .NET v2 and Windows 2003 Standard vs. Enterprise

[drew@sundawg.org]

I have an application I wrote in C#, which sends an email
periodically. The program works fine on my XP laptop, and on my
Windows 2003 Enterprise server. However, I've installed the
application on 2 Windows 2003 Standard servers where the program fails
to send the email and throws an exception.

Here's the relevant section of the C# code:

try
{
client.Send(msg)
}
catch (Exception e)
{
log.Log("Error sending email:" + e.Message.ToString())
}

And here's the error message from the 2003 Standard servers:

Error sending email:Command not implemented. The server response was:
Error: command not implemented

All 2003 Server machines sit on the same network, use the same
gateway, and are attempting to send to the same mail server (and email
address). Same versions of .NET and all servers are at SP2. Is there
some underlying DLL that isn't provided with Standard that is provided
with Enterprise that might cause this? Why would this same program
work flawlessly on XP?

Thanks,

Drew