Mobile number portability lets a subscriber keep their number when changing operator, so the prefix of a number no longer guarantees which network will receive the message.
Before portability, a number's prefix identified its operator. Now, in most countries, a number can belong to any operator. Routing therefore needs a portability lookup on the carrier side to find the current network; a route that ignores porting delivers to the wrong SMSC and fails or is delayed.
Portability also affects sender rules and prices when operators within a country behave differently. The route, not the sender, has to resolve this per message.
At SMSMeteor: porting is resolved on the route; the report names the operator that actually handled a message when a failure carries a reason.