We can't create mailer directly in DI container using the factory because it may fail in case
the plugin is not configured properly and prevent plugin from start.
The Mailer was previously injected because it contains lazy initialization.
The lazy initialization in the singleton Mailer service was dangerous,
because when used with some custom parameters it could cause that all services
that inject the Mailer from DI would use the custom config instead the default.
[MAILPOET-4115]
