Create Multi-Domain MTA (Postfix or other) on Ubuntu 20.04

I want to create a virtual Ubuntu 20.04 server on Digital Ocean or HostWinds with FQDN pf.<my domain>.com to support mail for that domain AND support mail for 2 other domains. This will be a SEND ONLY server. I want iptables to block all inbound traffic except SSH and allow any traffic from 3 IP addresses. DKIM and SPF must be configured. In addition to the configured working server, I need clear documentation for: 1 - Add new email address to one of the Domains 2 - Add a new Domain to the server including DKIM and SPF The full underlying requirement: This server will be used by applications on our server to send email for order confirmation, error reports, receipts, etc. The "FROM" address must be from different domains/accounts. The mail cannot be bounced due to SPF or DKIM. We control DNS for all these domains.