The Messaging Server SMTP server implements the Simple Authentication and Security Layer (SASL, RFC2222) protocol. SASL can be used with POP and IMAP clients to provide password-based access to your SMTP server. A typical usage for SASL is to permit mail relaying for external authenticated users. This solves the common problem posed by local users who use ISPs from home or while traveling. Such users, when connecting to your mail system, will have non-local IP addresses. Any relay blocking that takes into account only the source IP address will not permit these users to relay mail. This difficulty is overcome through the use of SASL, which enables these users to authenticate themselves. Once authenticated, the users are permitted to relay mail.