I found that the AuthorizedTo list has done more to stop spam and spam attacks than any other filter.  We are an ISP and were being hit hard with phishing spam attacks.  Once we decided to use the AuthorizedTO feature we found it not only cut the spam but allowed our spamfilter processor to move from heavy to relaxed.

After we realized the benefits of this filter we wrote a service that pulls valid email addresses from our ISP mail server (it uses an SQL database) and merges them with a manual list (for clients that have their own email server), sorts them, removes any duplicates and saves them in a text file for the Spamfilter. The service runs every five minutes if there are changes it saves a new AuthorizedTo file. With this service any new email address added to a clients hosted email account (even by the client) is added to the AuthorizedTo list without any effort on our part.

I just thought I'd share how we made this filter work for us. 

Dan,

We too have CatchAll Accounts within our mailserver and using the AuthorizedTo feature without any problems.  In the AuthorizedTo list just use an *@yourdomain.com as the catchall domains and it will still allow the other domains to have AuthorizedTo addresses.

Dan B