"Mailbox unavailable. The server response was: <xxx> No such user here" error with VS 2005 System.Ne

[JT]

Hi,

I've seen a couple of postings about this, but no resolution. I am
using Visual Studio 2005 and its System.Net.Mail.SmtpClient class to
send emails. I am instantiating it with the host (server) set to
either mail.[mydomainname] or the IP address. Both appear to act the
same.

If I send an email from Sales@[mydomainname] to an external email
address (my personal email address), I get the following error
message, "Mailbox unavailable. The server response was: <[my personal
email address]> No such user here".

If I send an email from my personal email address to my personal email
address I get the same message.

If I send an email from Sales@[mydomainname] to any other
[mydomainname] email address, the email goes through.

So it appears that I can only send emails to addresses that are in the
[mydomainname] domain.

Here's the kicker. If I send emails through Outlook using either the
IP address or the mail.[mydomainname] as the outgoing mail server, I
can send to all email addresses just fine.

Is there some IIS or mail server setting that is preventing me from
sending emails through a Visual Studio application to an "external"
email address?

Thanks,

JT