If you are seeing continuous problems, I'd love to take a look! My email address is on my profile.
In the case of Mailgun, you should be assigned an IP with a neutral reputation. For example, before dedicated IPs are reassigned we leave them dormant for at least a month, usually much longer, before assigning to a new customer.
Mailgun will do some chores for you, for a fee. If those chores are things you can do in your sleep, fine! Mailgun does nothing magic.