I'm trying out a hosted anti-spam solution. This service works by becoming my public-facing mail server (DNS MX record), and relaying non-spam mail to the mail server in my office.

Due to a configuration error, I lost about 15 hours of email--the host wasn't relaying for my domain. Email was bouncing with the message "553 Relaying is not supported".

Lesson learned (with thanks to Ezra Herman):  before switching DNS, use Telnet to test the connection to the host and make sure it is accepting email for my domain. Here's how to do it:

XFOR: Telnet to Port 25 to Test SMTP Communication
http://support.microsoft.com/kb/153119

The configuration is fixed, I sent myself an email using a Telnet session to the anti-spam host, so now I can update my DNS!