I was just informed by Mailgun support that it is not possible to configure the maximum number of messages sent per IP address per second to any specific IP address. My client is therefore unable to rely on the messages sent via their dedicated Mailgun IP address to their very own mail server, which is hosted by Tucows.

Also, it took me many months of submitting tickets, sending tests, and talking to Mailgun Support to trace this as the deliverability problem that I have been trying to resolve. This fact is not advertised anywhere that I can find and it was a hugely difficult process to finally discover that this is the root of the issue.

We may need to find a new mail host or sender host. It's a shame that two large corporations have policies that contradict each other. This makes it very hard to be your customer.

This is very frustrating.