[andrewjshults]

I've implemented similar systems twice before (social media at frid.ge - YC S'10, b2b adtech at ADstruc) and think that if you are doing onsite messaging, reply-by-email should be a core part of your messaging implementation. Users, especially non-technical ones, have an expectation that you can reply to any email (set the reply-to on your outgoing message to a mailbox you monitor if you want a cheap test). Depending on your product, reply-by-email can give you a fairly competent mobile experience on the cheap.

Highly recommend mailgun for this, since the routes + email cleaning (removing signatures, replies, etc.) makes this super simple to implement. At frid.ge we used sendgrid (mailgun didn't exist at the time) and spent a lot of time dealing with problems that mailgun has already solved (at least better than we ever did - stripping signatures without destroying the message body is very difficult).