Manual:$wgEnotifWatchlist

Details
Set to true to allow e-mail notification for watched pages. The option is now shown in user preferences and users can opt-in to receive one e-mail when someone else changes a page they are watching.

If $wgEmailAuthentication is false, this setting is ignored.

Each user can after tick the box "E-mail me when a page on my watchlist is changed" in Special:Preferences.