There could be two issues:

• Or it's a mail transfer agent issue.
  
• Or it's a spam related issue.
  

For the second point you could try this website: http://www.mail-tester.com/ it'll show you if the DNS records are fine.

I'm also in the process of "upgrading" my personal mail and learning more about the whole mail server architecture. My goal is to be able to setup a mail server with virtual users and a filter that will use GPG to encrypt mails stored according to the receiver.