Mail to SMPT server Causes error

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Okay I have a really weird issue. I'm not sure where to post this, but since
it's a .NET app I'll try posting it here first.

I've written a standard windows forms app to generate emails to a
distribution group. Code is clean. The smtp server is using the default port
25, and I'm providing credentials object to the smptclient class. When I try
to use the app from my client's smtp server I get the following exception
"There was an error in trying to braodcast. Syntax error, command
unrecognized. The server response was: Unauthorized relay msg rejected."

Now here's the real kicker... With the application still open, if I go to
Outlook and create a new account using the credentials for the account I'm
trying to send from, the application starts working, even if I don't
recompile. If I don't save the account in Outlook, I get the ability to send
for about an hour then I get the old exception again.

I've contacted the web host provider (Hostway) who says it has something to
do with my ISP provider (Comcast) and that on SpamHaus.com, they have comcast
IP addresses marked as dangerious, then they're telling me that it is
Comcast's fault.

Has anyone run into anything like this? I've written many dozens of
applications like this and have never run into this type of problem. I could
port to a web app, but would prefer not having a mail generation app on the
client's website even if it is secured.

Thanks in advance.
.

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