Can SMTP Errors be the Developer's fault?

Question

I have an error message getting returned to me which would appear to be something wrong with the Exchange set up. Is there a possibility that I'm doing something wrong? I have no idea where to to start to track this down:

The following recipient(s) cannot be reached:
  Customer Service Account on 6/3/2009 11:00 AM
         There was a SMTP communication problem with the
recipient's email server.  Please
contact your system administrator.
<fgdc.myservername.net #5.5.0 smtp;550 Requested action not taken: mailbox unavailable>

This is perhaps a ServerFault question, but I wanted to get some input as to whether it's even possible that there's something I can do to fix it in my code.

Site is Asp.Net C#, using URL Routing

Server is 2003, 64-bit and running Exchange 2003

UPDATE

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Turns out it was a layer of Spam protection. Figured out this was only happening for internal addresses and MIMESweeper looks to be throwing away the messages. They were coming from an external web server, but sending with an internal domain. Flags go up. Messages don't go.

Answer 1

There is nothing to fix - the only thing you can do is recover gracefully and log the error. The only thing to check is make sure you have the correct email address since the error is "mailbox unavailable".

Answer 2

My guess is that you have the wrong user specified. Particular versions of Exchange are very picky about usersnames/emails. If you put just the "full name" in as the recipient, you won't get anything. You have to have an exact Match AND with Exchange 2000 you also needed an X.400 address to have it work too.

There are so many things that can go wrong with the Exchange SMTP gateway that I really can't say what exactly your problem is. You're better off talking with the Mail admin, and turning on Message Tracking for that mailbox to find out what is going on under the hood.