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).
This is basically a nice tutorial on implementing two-way email messaging between machine and humans. Kudos for creative use of our "Email Routes" feature (disclaimer: I work for Mailgun).
The mailing list idea was a great idea. It helps a lot and makes me feel more comfortable when someone of similar skill level is able to help tackle a tough problem.
Your cohort was pretty active, and I think a lot of it had to do with the reply-by-email functionality. For example, I was able to just reply while on my cell without fumbling with mobile Safari. You should've seen earlier cohorts, a lot less discussion (sorry guys, if any of you are reading this!).
Author of the blog here. In my opinion the inbounding email workflow is very under-utilized. Email is one of the best user input channels - while most people do not keep a web page open all the time, they do check emails very often and knows how to use it. Posterous and TripIt are great examples of using this well when collecting user input/data is a high priority for their apps.
Let me know if you have any questions, or Mailgun in general - I have looked into a few different email service providers and happy to share perspectives
Typically there are 3 use cases for email services - transactional emails, campaign emails and receiving emails. You have the most options with it comes to sending transactional emails. But pay attention to some of the lower priced providers because they only help you to "send" emails, but not putting much effort that the emails are actually delivered - it is hard engineering with all the email provider's spam rules etc. The ones I recommend in this area are Mailgun and Postmark. Both have good reputation doing this well.
Then you have inbounding emails, which is newer and less providers do that. Both Mailgun and Postmark have it, and there are some newer players like cloudmailin, which I haven't used.
On the campaigning side, the biggest player is MailChimp with good reputation. I have used their service, but it feels like it's mostly built for marketers - Nice UI, a lot of templating choices, and your workflow is mostly in the browser. If that's your use case, they should serve you well.
As for as I know, Mailgun is the only email provider that has all 3, plus Mailing Lists and Storage (you can create mail boxes.. or basically build your own version of GMail on top of Mailgun).. For Tealeaf Academy (http://www.gotealeaf.com), we use delivering, receiving, mailing lists and campaigning, so Mailgun is what we picked. It also has a "closer to the metal" feel with their lower level APIs, which we like a lot.
I'd also recommend Postmark, if your app focuses on delivering and receiving transactional emails, and their pricing makes sense to you - I have seen some of their developer's posts and they know this stuff very well too.
I'd avoid some of the cheaper options, especially on transactional email delivery - it's a big deal (sending people receipts, notify credit card expiration, resetting password etc) and you want to stick with people who know how to do this.. don't cheap out
They can - we just need to create a course specific email address and let people email to that, and determine the poster either by sender email address (they will have to use the same email that they registered with us, and this is also less secure) or embed a user token in the "To:" field (more secure, but harder for people to remember)
For us, when people ask questions, they are likely on our site already, either working on homework assignments or watching lecture videos, so initiating a thread with email is less of a need.
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).