Make sure you have DMARC; DKIM and SPF setup.

Follow these instructions.

https://mediatemple.net/community/products/dv/204405554/how-do-i-enable-dkim-domainkeys-on-vps-hosting

SPF, DKIM & DMARC Explained: How To Set Them Up And Combat Fake Emails
https://snov.io/blog/how-to-set-up-spf-dkim-dmarc/