From https://gist.github.com/johnbillion/0a48021de5510c41d517

WordPress Emails

This document lists all the situations where WordPress sends an email, along with how to filter or disable each email.

This is accurate as of WordPress 4.8, and includes some upcoming changes in WordPress 4.9.

There are a few TODOs left. Please bear with me.

• Comments
• Change of Admin Email
• Change of User Email or Password
• Automatic Updates
• New User
• New Site
• Other

Comments

Comment is awaiting moderation

To:        Site Admin, plus post author if they can edit comments
From:      WordPress <wordpress@host>
Subject:   [%s] Please moderate: "%s"
Function:  wp_notify_moderator()
Pluggable: Yes
Filters:   comment_moderation_subject
           comment_moderation_text
           comment_moderation_recipients
           comment_moderation_headers
Disable:   Return false from notify_moderator filter
           Remove wp_new_comment_notify_moderator action on comment_post hook
           "Email me whenever" on Settings -> Discussion
           Overwrite the pluggable function

Comment has been published

To:        Post author
From:      WordPress <wordpress@host>
Subject:   [%s] Comment: "%s"
           [%s] Pingback: "%s"
           [%s] Trackback: "%s"
Function:  wp_notify_postauthor()
Pluggable: Yes
Filters:   comment_notification_subject
           comment_notification_text
           comment_notification_recipients
           comment_notification_headers
Disable:   Return false from notify_post_author filter
           Remove wp_new_comment_notify_postauthor action on comment_post hook
           Overwrite the pluggable function
           See also the hardcoded action added to wp_set_comment_status in wp_set_comment_status()

Change of Admin Email

Change of site admin email address is attempted (Prior to WordPress 4.9, this was Multisite-only)

To:        Proposed new email address
From:      WordPress <wordpress@host>
Subject:   [%s] New Admin Email Address
Function:  update_option_new_admin_email()
Pluggable: No
Filters:   new_admin_email_content
Disable:   Remove action on add_option_new_admin_email and update_option_new_admin_email hooks

Site admin email address is changed (WordPress 4.9+)

To:        Old site admin email address
From:      WordPress <wordpress@host>
Subject:   [%s] Notice of Admin Email Change
Function:  wp_site_admin_email_change_notification()
Pluggable: No
Filters:   site_admin_email_change_email
Disable:   Return false from send_site_admin_email_change_email filter

Multisite only: Network admin email address is changed (WordPress 4.9+)

To:        Old network admin email address
From:      WordPress <wordpress@host>
Subject:   [%s] Notice of Network Admin Email Change
Function:  wp_network_admin_email_change_notification()
Pluggable: No
Filters:   network_admin_email_change_email
Disable:   Return false from send_network_admin_email_change_email filter

Change of User Email or Password

User requests a password reset via "Lost your password?"

To:        User
From:      WordPress <wordpress@host>
Subject:   [%s] Password Reset
Function:  retrieve_password()
Pluggable: No
Filters:   retrieve_password_title
           retrieve_password_message
Disable:   Not possible

User resets their password

To:        Site admin
From:      WordPress <wordpress@host>
Subject:   [%s] Password Changed
Function:  wp_password_change_notification()
Pluggable: Yes
Filters:   wp_password_change_notification_email (WordPress 4.9+)
Disable:   Remove action on after_password_reset hook
           Overwrite the pluggable function

User attempts to change their email address (Prior to WordPress 4.9, this was Multisite-only)

To:        Proposed new email address
From:      WordPress <wordpress@host>
Subject:   [%s] New Email Address
Function:  send_confirmation_on_profile_email()
Pluggable: No
Filters:   new_user_email_content
Disable:   Remove action on personal_options_update hook

User changes their password

To:        User
From:      WordPress <wordpress@host>
Subject:   [%s] Notice of Password Change
Function:  wp_update_user()
Pluggable: No
Filters:   password_change_email
Disable:   Return false from send_password_change_email filter

User changes their email address

To:        User
From:      WordPress <wordpress@host>
Subject:   [%s] Notice of Email Change
Function:  wp_update_user()
Pluggable: No
Filters:   email_change_email
Disable:   Return false from send_email_change_email filter

Automatic Updates

Completion or failure of a background automatic core update

To:        Site admin / Network admin
From:      WordPress <wordpress@host>
Subject:   [%s] Your site has updated to WordPress %s
           [%s] WordPress %s is available. Please update!
           [%s] URGENT: Your site may be down due to a failed update
Function:  WP_Automatic_Updater::send_email()
Pluggable: No
Filters:   auto_core_update_email
Disable:   Return false from auto_core_update_send_email filter
           Return false from send_core_update_notification_email filter

Full log of background update results, sent when you are using a development version of WordPress

To:        Site admin / Network admin
From:      WordPress <wordpress@host>
Subject:   [%s] There were failures during background updates
           [%s] Background updates have finished
Function:  WP_Automatic_Updater::send_debug_email()
Pluggable: No
Filters:   automatic_updates_debug_email
Disable:   Return false from automatic_updates_send_debug_email filter

New User

Multisite only: An existing user is invited to a site from Users -> Add New -> Add Existing User

To:        User being invited
From:      WordPress <wordpress@host>
Subject:   [%s] Joining confirmation
Function:  wp-admin/user-new.php
Pluggable: No
Filters:   None
Disable:   Click the "Skip Confirmation Email" checkbox when adding the user

Multisite only: A new user is invited to a site from Users -> Add New -> Add New User

To:        User being invited
From:      [Network Name] <[network admin]>
Subject:   [%s] Activate %s
Function:  wpmu_signup_user_notification()
Pluggable: No
Filters:   wpmu_signup_user_notification_subject
           wpmu_signup_user_notification_email
Disable:   Click the "Skip Confirmation Email" checkbox when adding the user
           Return false from wpmu_signup_user_notification filter

Multisite only: A new user account is created

To:        Network Admin
From:      WordPress <wordpress@host>
Subject:   New User Registration: %s
Function:  newuser_notify_siteadmin()
Pluggable: No
Filters:   newuser_notify_siteadmin
Disable:   Filter registrationnotification option value
           Remove action on wpmu_new_user hook
           Toggle "Registration notification" in Network Admin -> Settings

Multisite only: A user has been added, or their account activation has been successful

To:        User being added
From:      [Network Name] <[network admin]>
Subject:   New %s User: %s
Function:  wpmu_welcome_user_notification()
Pluggable: No
Filters:   update_welcome_user_subject
           update_welcome_user_email
           See also "Welcome User Email" setting in Network Admin -> Settings
Disable:   Click the "Skip Confirmation Email" checkbox when adding the user
           Return false from wpmu_welcome_user_notification filter
           Remove action on wpmu_activate_user hook

A new user is created (@TODO: needs a lot more information here)

When a new user is created, two emails are sent from the same function. One to the site admin:

To:        Site Admin
From:      WordPress <wordpress@host>
Subject:   [%s] New User Registration

and one to the newly created user:

To:        User being added
From:      WordPress <wordpress@host>
Subject:   [%s] Your username and password info

Details:

Function:  wp_new_user_notification()
Pluggable: Yes
Filters:   wp_new_user_notification_email_admin (WordPress 4.9+)
           wp_new_user_notification_email (WordPress 4.9+)
Disable:   Remove wp_send_new_user_notifications action on register_new_user hook
           Remove wp_send_new_user_notifications action on edit_user_created_user hook
           Remove wp_send_new_user_notifications action on network_site_new_created_user hook
           Remove wp_send_new_user_notifications action on network_site_users_created_user hook
           Remove wp_send_new_user_notifications action on network_user_new_created_user hook
           Overwrite the pluggable function

New Site

When WordPress is installed, and when a site is added to a Multisite network

To:        Site Admin
From:      WordPress <wordpress@host>
Subject:   New WordPress Site
Function:  wp_new_blog_notification()
Pluggable: Yes
Filters:   None
Disable:   Overwrite the pluggable function

Multisite only: New site created from Network Admin -> Sites -> Add New

To:        Network Admin
From:      Site Admin <[network admin]>
Subject:   [%s] New Site Created
Function:  wp-admin/network/site-new.php
Pluggable: No
Filters:   None
Disable:   Not possible

Multisite only: User registers for a new site

To:        Site Admin
From:      [Network Name] <[network admin]>
Subject:   [%s] Activate %s
Function:  wpmu_signup_blog_notification()
Pluggable: No
Filters:   wpmu_signup_blog_notification_subject
           wpmu_signup_blog_notification_email
Disable:   Return false from wpmu_signup_blog_notification filter
           Remove action on after_signup_site hook

Multisite only: User activates their new site, or site added from Network Admin -> Sites -> Add New

To:        Network Admin
From:      WordPress <wordpress@host>
Subject:   New Site Registration: %s
Function:  newblog_notify_siteadmin()
Pluggable: No
Filters:   newblog_notify_siteadmin
Disable:   Filter registrationnotification option value
           Remove action on wpmu_new_blog hook
           Toggle "Registration notification" in Network Admin -> Settings

Multisite only: User activates their new site, or site added from Network Admin -> Sites -> Add New

To:        Site Admin
From:      [Network Name] <[network admin]>
Subject:   New %s Site: %s
Function:  wpmu_welcome_notification()
Pluggable: No
Filters:   update_welcome_subject
           update_welcome_email
           See also "Welcome Email" setting in Network Admin -> Settings
Disable:   Return false from wpmu_welcome_notification filter
           Remove action on wpmu_activate_blog hook

Other

Multisite only: Site admin deletes site from Tools -> Delete Site

To:        Site Admin
From:      WordPress <wordpress@host>
Subject:   [ %s ] Delete My Site
Function:  wp-admin/ms-delete-site.php
Pluggable: No
Filters:   delete_site_email_content
Disable:   Not possible