Note that while there is a fair amount of commonality amongst
    the commands accepted by many of the different mail servers,
    differences also exist. Do not expect PMDF mail server commands
    to work with other mail servers and do not expect other mail
    server commands to work with PMDF mail servers. When you are
    unsure of what sort of mail server you are dealing with, the
    first order of business should be obtaining help information for
    that server. Often this can be done by sending the command HELP
    to the server.