Yes, there is a simple Authenticate module that will make it easy to hook into any other users database -- everything from an SQL table, to a flat file system, to something else.
Also, users can be auto created in the program, so you can just install the program, and tell all your users they now have free email. When they log on for the first time their account gets automatically setup.
With a little work, it's also possible to pass sessions back and forth so that the users don't need to log in twice (although this usually requires a little scripting knowledge).
Cheers,
Alex
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