Manually run the new unit test and see green lights:
arc unit b/externals/mimemailparser/__tests__/PhabricatorExternalMimeMailParserTestCase.php
Double-check that the new class is already recorded:
arc liberate
Just as extra care, re-apply the same test plan of:
D25839
So, for example, run this, and see no exceptions:
./scripts/mail/mail_handler.php < src/applications/metamta/externals/__tests__/data/test_accents.mbox