Almost certainly a large part of your problem is that your individual IP address is in a shared net block that has other neighbors that have historically been a source of spam.
Unless you can get hosting at an ISP that does not sell low budget, commercial virtual private server, virtual machine or dedicated hosting to random people with $20 and a credit card, this will be an ongoing problem.
Unless you can get hosting at an ISP that does not sell low budget, commercial virtual private server, virtual machine or dedicated hosting to random people with $20 and a credit card, this will be an ongoing problem.