I don’t believe it’s possible to completely disable them, but nobody should be getting 60 messages/day unless you want it to be that way (in my case, my Discourse forum replaces an old Yahoo group that was primarily used as a mailing list, so I enabled mailing list mode by default for my users). “Normal” email notifications would send you an email if you’re mentioned, or if a reply is made to one of your posts, and you aren’t on the forum at the time. The normal notification settings will also send an email digest to users occasionally when they haven’t visited the forum in a while (the details are configurable).
So, if you don’t want to be sending a ton of emails, and your users aren’t expecting Discourse to operate like a mailing list, disable mailing list mode (search in the site settings for “mailing”), and make sure the other notification settings are sensible.
None of which actually answers your question, but might still be helpful nonetheless. But to your actual question, I’ve been happy with Mailgun. Most of the time, my usage is low enough that I don’t get a bill (they don’t bill for less than $1/month).