1. Misconfigured servers: This is the hardest part. Rule number 1: keep it as small as possible. Disable everything, and then only enable what you need. Rule 2: READ, I mean read every document, blog post etc you can find about e-mail, email attacks, etc.
2. IP address: The normal address you get from Digital Occean or Linode for example, are normally horrible as a lot of spam has been sent from them. If your lucky it might work, but check the lists for those IPs. Try to change if you can, until you find one with low scores. Also, I dont know if this helped, but i had my domain on both Gmail and Outlook servers for a while before migrating to my own servers. I dont know if this helped me to not get stuck in their filters etc? Or I was just lucky i guess :)
I finally moved because I did not have the energy to keep up with the server etc, and when i noticed that i had not even logged on to the server for 3 months, i just moved it to gmail. I am however looking at migrating away from google, as i don't like the web interface (use it from the iPad now). So who knows where I'll end up next.