Inventory inbound and outbound connectors and record:
- direction;
- purpose;
- owner;
- domains;
- source/destination IPs;
- certificate/TLS restrictions;
- smart hosts;
- validation status.
Compare each connector with current mail-flow diagrams and DNS.
A connector left behind after a gateway migration can create a hidden dependency or unexpected trust path.
Test before deleting. Old applications and partners often reveal themselves only when the supposedly unused connector disappears.