Hi,

I'm writing a VB.NET 2.0 WinForms app for our company that needs to send
email externally from ourselves (to customers).  We are using Exchange 2003.
My VB code is using the System.Net.Mail.SmtpClient namespace library.

I have come against a problem getting Exchange to allow externally addressed
messages to be sent (relayed from internal to external).

I can happily send messages to any internal recipient (e.g. myself for
testing), but as soon as I try to send to an external address (e.g. my home
email address) I get a 5.7.1 error response; full text = "Mailbox
unavailable. The server response was: 5.7.1 Unable to relay for alec@<...>"
(where <...> is my home email domain).

Looking at the Exchange relay settings, I have the following settings:
Select which computer may relay through this virtual server: Only the list
below.
Allow all computers which successfully authenticate to relay, regardless of
list above (checked).

I have one IP in my allowed computers list, which is my internal LAN IP of
my webserver, which enables it to send mail from various applications.

I _could_ add my client PC to this list (and indeed this works) but I don't
want to be continually changing IP's when users change machines or the DHCP
provided address changes.

I have tried the alternative of not checking the "Allow all computers which
successfully authenticate...", creating an Active Directory security group
which will be granted Relay rights into which I add another security group
which is the application users, then adding this to the specifed Users list
and checking the Submit and Relay boxes, applying all changes, and
restarting the virtual SMTP server.

This gets the same 5.7.1 error.

Other than using the specific IP address approach (which as mentioned, I'd
rather not), what else can I do?
Have I missed something (not) obvious?

Thanks for any help.

Al