SMPP, Short Message Peer-to-Peer, is a protocol from the 1990s for exchanging SMS at high volume between an application and a carrier or aggregator over a persistent connection. It is still the workhorse of wholesale SMS, and some providers offer it alongside a REST API. It carries SMS and MMS only; RCS is sent over HTTPS through the RBM API, not over SMPP.
How it works
An application opens a bind to the provider's SMPP server as a transmitter, a receiver or both, then submits messages and receives delivery reports over that open session rather than one HTTP request at a time. The persistent connection is what makes it efficient for millions of messages an hour, and also what makes it more work to operate: connection management, throttling, encoding and the version differences between SMPP 3.3, 3.4 and 5.0 are all the sender's problem.
When it is the right choice
SMPP suits carriers, aggregators and very high-volume SMS senders with engineering teams that already run it. For a business programme, a REST API with webhooks does the same job with far less to maintain, and it is the only route to RCS, because the rich message types, capability checks and read receipts have no SMPP equivalent.
On SimplyRCS
Sending is through a REST API and a no-code app, with delivery, read and reply events returned by webhook. The RCS API overview and the webhook guide cover the integration.