During a couple of recent POCs, this topic came up so I thought I"d put the reference here so I can find it easier next time...maybe it will help you find it easier as well... Add users from multiple forest domains </steve>

There is growing interest in using email-enabled lists in SharePoint...not a lot of enterprise examples yet and so not a lot of best practices. If you are considering this, check out the following: Email Enabled Lists and Inbound Email E-Mail Records...

For those interested in using ADAM as a user authentication store for SharePoint 2007 you might want to check these out. They will help with understanding as well as configuration. How to use WSS v3 with ADAM for user authentication SharePoint PeoplePicker...