The SMS standards organization that created the specification has disbanded, and there are plans for their web site to be shut down.
This doesn't mean that SMPP is not still important for SMS. It remains the de-facto standard for application based SMS messaging.
It's just that the SMPP v3.4 specification was published in 1999, and it really hasn't been updated since.
The group did produce a successor specification, SMPP v5.0 ... but it was too different, and never really caught on in the marketplace. This leaves SMPP v3.4 as the de-facto industry standard.
More information:
http://en.wikipedia.org/wiki/Short_Message_Peer-to-Peer
SMSFORUM
SMS Gateway:
https://turbosms.ua/gateway.html