TYPO3 Core

Currently, when using $GLOBALS['TYPO3_CONF_VARS']['MAIL']['transport'] = '<classname>'; and/or $GLOBALS['TYPO3_CONF_VARS']['MAIL']['transport_spool_type'] = '<classname>';, the class in question is instantiated via makeInstance() along with constructor arguments, which does not allow to use a configured service here which itself allows for DI. this also blocks extending AbstractTransport which does a lot of useful logic already you don't want to reimplement.

The core should use a signaling interface here like MailSettingsAware which passes the mail settings to the service (if needed) instead of passing those along as constructor arguments, especially since those are available through $GLOBALS anyway...