Inventory users and aliases
Document every mailbox, alias, shared mailbox, forwarding rule and group before changing MX records. It is much easier to validate a migration against a known list.
Protect DNS access
Confirm access to the domain registrar and DNS zone, record current MX/SPF/DKIM/DMARC settings, and reduce TTL in advance when an organized cutover is required.
Validate after cutover
Test inbound and outbound mail, external delivery, aliases, mobile clients, Outlook profiles and authentication records. Keep the old service accessible during the agreed validation window when possible.