15 files declare their use of AssertMailTrait

AssertMailTraitTest.php in core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php
ConfigTranslationUiTestBase.php in core/modules/config_translation/tests/src/Functional/ConfigTranslationUiTestBase.php
ContactPersonalTest.php in core/modules/contact/tests/src/Functional/ContactPersonalTest.php
ContactSitewideTest.php in core/modules/contact/tests/src/Functional/ContactSitewideTest.php
EmailActionTest.php in core/tests/Drupal/KernelTests/Core/Action/EmailActionTest.php
MailCaptureTest.php in core/tests/Drupal/FunctionalTests/MailCaptureTest.php
SiteMaintenanceTest.php in core/modules/system/tests/src/Functional/System/SiteMaintenanceTest.php
UserAdminTest.php in core/modules/user/tests/src/Functional/UserAdminTest.php
UserCreateTest.php in core/modules/user/tests/src/Functional/UserCreateTest.php
UserLoginHttpTest.php in core/modules/user/tests/src/Functional/UserLoginHttpTest.php
UserLoginTest.php in core/modules/user/tests/src/Functional/UserLoginTest.php
UserMailNotifyTest.php in core/modules/user/tests/src/Kernel/UserMailNotifyTest.php
UserPasswordResetTest.php in core/modules/user/tests/src/FunctionalJavascript/UserPasswordResetTest.php
UserPasswordResetTest.php in core/modules/user/tests/src/Functional/UserPasswordResetTest.php
UserRegistrationRestTest.php in core/modules/user/tests/src/Functional/UserRegistrationRestTest.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.