Issue #2187495 by Les Lim, longwave: Use plugin system for MailInterface classes.

core/core.services.yml

core/includes/mail.inc

core/lib/Drupal/Core/Annotation/Mail.php

core/lib/Drupal/Core/Mail/MailFactory.php

core/lib/Drupal/Core/Mail/MailInterface.php

core/lib/Drupal/Core/Mail/MailManager.php

core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php

core/lib/Drupal/Core/Mail/Plugin/Mail/TestMailCollector.php

...es/simpletest/lib/Drupal/simpletest/InstallerTestBase.php

.../modules/simpletest/lib/Drupal/simpletest/WebTestBase.php

core/modules/system/config/system.mail.yml

.../modules/system/lib/Drupal/system/Tests/Mail/MailTest.php

core/modules/system/system.api.php

...stem_mail_failure_test/Plugin/Mail/TestPhpMailFailure.php

...les/user/lib/Drupal/user/Tests/UserCreateFailMailTest.php

core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php