8 files declare their use of MailgunHandlerInterface in Mailgun 8

mailgun.install in ./mailgun.install

Contains install/uninstall stuff.

MailgunAdminSettingsForm.php in src/Form/MailgunAdminSettingsForm.php

MailgunAdminSettingsFormTest.php in tests/src/Functional/MailgunAdminSettingsFormTest.php

MailgunFunctionalTestBase.php in tests/src/Functional/MailgunFunctionalTestBase.php

MailgunKernelTestBase.php in tests/src/Kernel/MailgunKernelTestBase.php

MailgunMail.php in src/Plugin/Mail/MailgunMail.php

MailgunTestEmailForm.php in src/Form/MailgunTestEmailForm.php

SendMailBase.php in src/Plugin/QueueWorker/SendMailBase.php