The documentation → already states:
You don't need the Mail System module, as Symfony Mailer already includes similar features.
However, it's not 100% clear what happens if you already have Mailsystem installed. I tested, Mailsystem is simply overridden / ignored, causing my mail from Views setup to stop working. (That happened because Views Send, that I maintain, doesn't support Symfony Mailer yet. Will fix.) My point is that enabling this module has immediate consequences.
Update docs and decided if it is useful to mark conflict.
Fixed
1.4
Documentation
Sure you can edit the docs to clarify.
It cannot be a conflict because of migrating from swiftmailer (or similar) to this module. Temporarily you need both this and mailsystem at the same time.
Automatically closed - issue fixed for 2 weeks with no activity.