I have been managing my own mail server for 6 years now, and enabling and configuring all the mail security features (DKIM, SPF, etc), and keeping up with them, is indeed the most difficult part. I regularly check my mail server against tools like this:
https://mxtoolbox.com/diagnostic.aspx. If you just do one thing wrong, you will end up on some black list from which it is very difficult to get off. So far, I did not have any major problems, but after the initial default installation of Postfix in 2014, all my test mails to Google Mail went straight to Junk.
My server is at Hetzner, and there are some blacklists which block Hetzner IPs by default. You have to contact them, explain that your server is not a spam server, and hope to get whitelisted. In my case, it worked. T-Online in Germany also blocked my IP as well, but they were amazingly quick to whitelist it after I send them an email. They apologized and explained that they also block mail from Hetzner IPs by default.