12 files declare their use of AssertMailTrait in Drupal 10

AssertMailTraitTest.php in core/tests/Drupal/KernelTests/Core/Test/AssertMailTraitTest.php

ConfigTranslationUiTest.php in core/modules/config_translation/tests/src/Functional/ConfigTranslationUiTest.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

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

UserMailNotifyTest.php in core/modules/user/tests/src/Kernel/UserMailNotifyTest.php

UserPasswordResetTest.php in core/modules/user/tests/src/Functional/UserPasswordResetTest.php

UserPasswordResetTest.php in core/modules/user/tests/src/FunctionalJavascript/UserPasswordResetTest.php