Super helpful thanks. However the output from the debug is nearly as crytpic 
24/1 13:01:36 [7499] - warn: Missing translation "email:digest."
24/1 13:01:36 [7499] - warn: [emailer.mailgun] Unable to send digest
email to uid 1!!
24/1 13:01:36 [7499] - error: [emailer.mailgun] (Forbidden)
24/1 13:01:36 [7499] - error: [plugins] filter:email.send, Forbidden
Then for the sake of it I was wondering if it just didn't like sending an email to the admin. So I created a second user and see much of the same
24/1 13:03:19 [7499] - warn: [emailer.mailgun] Unable to send welcome
email to uid 2!!
24/1 13:03:19 [7499] - error: [emailer.mailgun] (Forbidden)
24/1 13:03:19 [7499] - error: [plugins] filter:email.send, Forbidden
24/1 13:03:19 [7499] - warn: [emailer.mailgun] Unable to send registration_accepted
email to uid 2!!
24/1 13:03:19 [7499] - error: [emailer.mailgun] (Forbidden)
24/1 13:03:19 [7499] - error: [plugins] filter:email.send, Forbidden
Looks like permissions to me. But I'm not comfortable enough about the nodebb layout on my server to be sure.