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 -> ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. 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=4504h,AX=4505h