Email response setup

So we send outboung email using gitlab ee 17.2.1.

I am slightly confused about inbound email, which my group wants to use to get back approvals without going to the web page. I read the email doc but my question is:

Something made me think that gitlab could receive old fashion mbox format email. So is an imap server required? Or can it read Maildir format?

I can set imap servers up, I just want to make sure I am going down the right path.