You can easily list databases in the mail subdirectory, but that's hardly definitive. (A mail database can be in any subdirectory.) You can also create a list of databases that use the Mail template. That's hardly definitive either (any database can use the Mail template.) You can go through the Names.nsf (and any secondary address books) and look at the databases listed in the various Person documents, but that's about it.
Further, there's no concept of "connected". The code in one database can try to open another, but chances are that "connection" is fleeting and during runtime. And the code can be @Formula, LotusScript, Java, or even a Web service.
So as I see it, you have two options, 1) Comb through all the design synopses of all the databases and find all of them that reference the mail databases, or 2) Perform the "Scream" test. Make the mail unavailable and see who screams.