A grey route delivers application SMS through channels not agreed with the destination operator (SIM farms, exploited interconnects), cheaply and unreliably, with fake receipts and sender replacement as common side effects.
Grey routes exist because operators charge for A2P termination and some intermediaries find ways around it: consumer SIMs in bulk, roaming loopholes, unmetered interconnects. The traffic is cheap until the operator notices, then it is blocked, sometimes with the sender's numbers and names on a blacklist.
Symptoms: suspiciously low prices for a hard destination, delivery reports that always say delivered, sender IDs quietly replaced, messages arriving hours late or not at all. A handset test with a few real phones shows the truth faster than any report.
At SMSMeteor: routes are measured on real delivery to handsets and swapped when they degrade; the guide gives a test protocol you can run against any provider, including us.