The Enotif system provides email notifications when a user talk page is edited, or when a page on their watchlist is changed.

In MediaWiki's prehistory, this was implemented in Extension:Email notification, but it has been integrated ages ago. Starting with MediaWiki 1.44 the Enotif system is being replaced by Manual:Notifications.

When Extension:Echo is installed, it overrides the user talk page notification, and optionally overrides the watchlist notifications.

Documentation:

• Help:Watching pages#Email notification (for end-users)
• Manual:Configuration settings#Email notification (Enotif) settings (for site administrators)