What are you using for user 2 user direct messaging?

Hi Guys,

I'm working on some side projects and one useful feature would be for users to be able to DM one another so that they don't have to publicly publish their email address.

I don't want to build this myself as its fairly time consuming and these ideas are just at the MVP stage. What are some good off the shelf, inexpensive options for DM that I could get started with straight away?