*Repost of verbatim description:*

At the moment it is not possible to put a new E-Mail transport class into an extension. Even if bin/php/ezpgenerateautoloads.php shows up a message that new transport classes will be taken from the extension directory, that is not the case.
In lib/ezutils/classes/ezmailtransport.php (line ~124) the code always tests for file existance in lib/ezutils/classes/ and rejects new transport classes if that file does not exist.