what is it?
SASL stands for Simple Authentication and Security Layer, which is a mechanism used to expand the authentication capability of C/S mode. In Postfix, you can use SASL to determine whether a user is authorized to use the forwarding service, or to identify who is using your server.
SASL provides a generic way to add authentication support to connection-based protocols, while XMPP uses a common XML namespace to meet SASL's needs.