• SPF record
  • Most important, make sure to add both the ipv4 and ipv6 of your server!! 
  • Use -all at the end of your record!
• DKIM will enhance delivery, however SPF will give you the most gain in delivery success.
• DMARC is an extra record which can improve even more, but it's gain in delivery success is less than SPF and DKIM.
  • However it's quite important from a security point of view (avoid abuse by external parties).
  • The most basic DMARC txt record to create, which will enforce SPF: v=DMARC1; p=reject; aspf=s;
• In doubt: Always use https://www.mail-tester.com/
  • Make sure that you use the exact same method as the one your experience problems with when sending the mail to mail-tester.