We have had two separate instances of Mailgun blocking our account without reaching out, forcing us to upgrade the account to have access to chat support in order to resolve the issue in a reasonable time. They seem to only respond tickets once a day at best and make their best effort to wiggle out of any effort to help you get back to sending in any haste.
The first time was a mistake on their part when they switched plan tiers and our payment was not processed correctly, they fixed it entirely on their end and did not admit to the problem in the slightest. After this event we made sure to have an intermediate queue on our end in order not to have emails sent into oblivion.
The second time one of our users mass reported a bunch of old emails at once which triggered something that instantly blocked us. Even if we have very low reporting in general (no marketing emails) a single user was able to effectively kill sending of emails to all other users without any forewarning.
After that we forced ourselves out of the platform. When we look for a transactional email sending service, we expect at least an effort on the service to keep the api working while we resolve any issues that arise in a timely manner.