2 functions implement hook_mail()

Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.

mail_html_test_mail in core/modules/system/tests/modules/mail_html_test/mail_html_test.module

Implements hook_mail().

user_load_by_mail in core/modules/user/user.module

Fetches a user object by email address.