Great stuff. Meteor is getting more and more compelling as a platform. At this point, I'd seriously consider using it for a new project.
One thing I'd be slightly concerned about is the lack of a serious server-side rendering option, though. The "spiderable" package is a bit of a hack (as they themselves admit).
Mailgunner here: while I am not deeply familiar with Meteor technology, if the framework allows you to receive an HTTP POST from a 3rd party domain to a publicly available URL (with HTTPs or without), then yes - you can receive email messages right into your Meteor app.
Yes, you can use any SMTP service you want with Meteor, including Mailgun. It's just when you use meteor deploy, your app is automatically configured with Mailgun. (note- I work at Mailgun).
One thing I'd be slightly concerned about is the lack of a serious server-side rendering option, though. The "spiderable" package is a bit of a hack (as they themselves admit).