Exchange 2007 rejects POPcon emails with "550 5.7.1 Unable to relay". What do I do?

This error message indicates that your Exchange 2007 server is not accepting the email because it thinks it is not the authoritative server for the domain of the recipient and assumes it should relay the mail back to the internet (and rejects that).

You need to add your external internet domain name to the list of accepted domains in the Hub Transport:

Open the Exchange Management Console
Expand the Organization Configuration, select Hub Transport, and then in the work pane, click the Accepted Domains tab
In the action pane, click New Accepted Domain. The New Accepted Domain wizard appears.
On the New Accepted Domain page, complete the following fields:
- Name Use this field to identify the accepted domain in the user interface. You can type any name that you want.
- Accepted Domain Use this field to identify the SMTP namespace for which the Exchange organization will accept e-mail messages, i.e. "yourdomain.com"
After you complete these fields on the New Accepted Domain page, select Authoritative Domain to set the accepted domain type
Click New.
On the Completion page, click Finish.