Personally I found that one of the biggest factors causing me problems was nearly invisible, and that was my servers IP address.
I was using a cheap, popular host whose IP addresses had apparently been used by spammers in the past. No matter how much I corrected settings and tried different things, my email was always blocked.
Then I moved to a more obscure (green energy!) host with clean IP's and... everything has just worked ever since.
#SelfHosting #Email