As far as I can tell, Mailgun's free tier now only allows sending to pre-authorised email addresses - essentially a test function - rather than any email address a commenter might use. The cheapest tier upgrade available through Heroku is $19 a month, which is much more than the "subscribe to comments" feature is worth for me. I have therefore disabled the feature on my website.

In future it would be nice if other mail sending solutions were available instead of just Mailgun.