[kh99]

If it's in another database, then you'd need to know where (like the table name and the field name in the table). Then you'd need to have the information to connect to that database (similar to what you put in the config.php for vbulletin. Wordpress must have something similar). From that point, the php code to do a query on the database is probably available in a lot of places on the net.

Another option might be to find a wordpress development site and see if someone there can tell you how to find out if an email address is in the database.

But to be honest, it kind of sounds like you might want to consider the option of paying someone to do it, unless the above sounds like something you're insterested in learning, and you have the time.