Keep addresses recognisable
Add addresses customers already use for support and label them consistently. Each connected address has its own forwarding and sending configuration.
Check connection state
Review forwarding, DNS, and test controls for every mailbox. A mailbox can receive successfully before its sending domain is fully verified.
Test after changes
Whenever the provider, forwarding rule, or DNS changes, send a new inbound message and a reply. See Email delivery troubleshooting if either direction fails.