We finally traced down why our customers have not been reliably getting emails sent by our web server. It turns out that this started happening after Exchange 2007 SP1 got installed. Basically email sent from our web server, relayed through our exchange server, was getting blocked by Exchange’s content filtering.

This is the exception that was getting thrown by our call to System.Net.Mail.SmtpClient.Send(MailMessage message).

System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering.

Here are the steps we followed to fix the issue.
– Open Exchange Management Console
– Expand Organization Configuration
– Select Hub Transport
– Select the Anti-spam tab
– Double-click Content Filtering
– Select the Custom Words tab
– Specify some word that is included in every email sent out to prevent
– Click Apply

Now our webserver can reliably send emails!