Currently, there are two places in Special:Preferences with settings about what mail you wish to receive:

• for core, in mw-prefsection-personal
• for Echo, in mw-prefsection-echo

That makes somewhat complicated to find how to disable watchlist mails (core): you try the notifications panel (nope, only Echo), the watchlist panel (nope) and can't see the setting.

We offer an unified mw-prefsection-notifications tab instead, used both by Echo and core. Future extensions handling notifications should also use this panel.