When it comes to sending and receiving emails, there are a few things that you need to make sure you have set up in your DNS in order to guarantee email flow.
Email Best Practice Recommendations
If you do not use any kind of security/spam system, I would suggest using Proofpoint. Email security, encryption, and more.
Proofpoint, Inc. is an American enterprise cybersecurity company based in Sunnyvale, California that provides software as a service and products for email security, identity threat defense, data loss prevention, electronic discovery, and email archiving.