Quick backstory: We've used Mailgun for email delivery since ~2014. Sent hundreds of thousands of emails in that time. No mailing lists, all user provided emails (account creations, receipts, password resets, confirmation emails) etc. We're on the 'pay per use' plan. Average > 98.5 % delivery rate. We average maybe 1000 emails per day.
Suddenly without prior notice we get an email: "Your account xxx has been temporarily disabled". Total blackout of all our email delivery.
Takes multiples emails and support ticket to get re-instated. Apparently we went above some "complaint rate threshold is 0.05% or lower" threshold, ad boom auto-disable. We now think (but guessing) our own users were complaining about contact form message spam getting through (as it seems google v2 recaptcha is easily spammed these days).
Not faulting MG for having spam protections in place, that's great. One one think after being a client for nearly a decade of clean email sending, you'd at least get a notice about unusual activity before a blanked account deactivation.
/rant
Any alternatives folks have had good relations/practice with?
We now use their API to forward any complaint to us to doublecheck, we became paranoid to keep the number lower.
We're very happy with postmarkapp.
Somebody was using random IPs, but real email addresses to signup. We don't know why, it never led to pageviews later. Maybe it was an attempt to check which user (email address) clicks the link in the confirmation email of a service they haven't signed up for. I can understand if such users click 'this is spam' which then gets reported back by the big email providers to postmarkapp.