We have some code in production that sends e-mails. Pretty common, I imagine. However I used the app today and received this message:

Unexpected Error: The message could not be sent to the SMTP server. The transport error code was 0x80040217. The server response was not available.

Wow, great error message, .NET! After a few minutes searching on google I found out that means that your SMTP username/password were denied. Thanks for the help, Microsoft!