Transport rule
A mail-server rule that changes messages in transit, sometimes used to stamp signatures.
Transport rules (called mail flow rules in Exchange Online) act on messages as they pass through the mail server. They can append text, such as a disclaimer, to every outgoing email.
Server-side signature products route mail through their own service to insert rich signatures. That reaches messages sent from any device, but it means mail passes through another company’s infrastructure.
Sendoff does not use transport rules or reroute mail. Signatures are placed in the email client itself.