INT 16 - Shamrock Software EMAIL - SPECIFY COMMAND-WORD FOR USER FUNCTION
        AX = 4503h
        DL = port number (01h = COM1)
        DH = maximum execution time in clock ticks (00h = 5 seconds)
        ES:BX -> ASCIZ string with new user command-word
Return: AX = 4D00h if EMAIL installed on specified port
Notes:  a single user command (consisting of only uppercase letters and digits)
          may be defined, and remains valid until it is overwritten or the
          EMAIL program terminates; the user command must be activated by
          calling AX=4504h at least once.
        an existing command word may be redefined with this function
SeeAlso: AX=4505h