One of the biggest challenges in setting up a new email server no a VPS like Digital Ocean or Linode, is getting the server IP address off all the DNS block lists and getting everyone to accept your mail as not spam. Setting up SPF and DKIM records seems to help a lot. After this, Google accepted my mail. Microsoft has been the only real problem as they blocked the IP address of the new mail server. It took about a week of repeatedly sending requests to their delist email, answering their questions, etc. Finally, they seem to have removed the block. An associate also said that Linode support can help with this if needed.