Domains
Email on a domain
MX, SPF, DKIM and rDNS — Steady DNS vs a mail host.
Website plans
Business email on a website subscription is part of that product. Use the mail hostnames we give you; do not point MX at a VPS unless we said the mailbox lives there.
Your own mail server
You need MX on @, SPF TXT, DKIM TXT, and usually DMARC TXT. rDNS on the sending IP should match the HELO name. Port 25 outbound may be restricted; check before you promise outbound mail from a new VPS.
Google / Microsoft 365
Follow their MX and TXT records exactly. Remove old MX records. Keep TTL moderate during the cutover.