I agree 100% on this, and it should be a global setting option.
I'm thinking about disabling email entirely, as people could be messaging sensitive information in MM that they would never want to be in some random email system that we have no control over.
end users don't think about it, even though they should. If I just "DM you on mattermost" it will just stay there, and therefore is secure enough. the email containing the detail of a message is a problematic leak IMO.