Forum Moderators: phranque
I'd provide a link to disable the emails of course if they didn't wish to receive it.
Basically it would involve getting a list of users who have not logged in in X number of weeks/months and sending them an email saying:
Dear Suchnsuch,
Your account at Foobar has the following updates. Go here to check them out. Click here if you don't want to receive any more emails.
We, as do many sites, do the same thing. We offer options for users to be updated when comments are posted to their threads, or when they get an inbox message, or a notification, whatever. But they have to opt-in.