Hello. I am administrator with 10 years experience.
I can help you with your email server. What mail server you use?
I'll check and configure all neccessary DNS records for you -mx, sfp, dmarc, dkim (if your mail server support it).
I have such experince with mail configuring at many servers include cpanel /whm too.
Send test email to address this page www,[login to view URL] and show me results page.
I will need ssh acceess to server and access to DNS panel to help you.