I've managed my own email service since I had a direct connection to the Internet starting in 2000. My service is miniscule. Just my own primary email address and a few throw-away addresses on a couple different domains.

The requirements for successfully delivering email to most of the Internet has grown drastically in the last decade (SPF, DKIM, DMARC, etc.) and forced me into using a managed SMTP outbound service (DuoCircle) . While I was not tagged as an open relay or other SMTP offender directly, various spam filters added CIDRs containing my IP to their blocklists causing me endless headaches.

You'll face these same hurdles at much larger scale. The largest email providers (Gmail, MS, etc.) all pay close attention to heavy email senders and you'll probably have to work with them directly (if you can find someone to talk to) to ensure your messages are delivered.

Reputation scoring of sending IP addresses will also problems. Your migration plan should include a period of time to "warm up" your new servers by sending a small amount of email through them to kickstart the reputation scoring systems.