INT 21 - LANtastic - SEND UNSOLICITED MESSAGE
AX = 5F98h
DS:SI -> message buffer (see below)
Return: CF clear if successful
CF set on error
AX = error code
Note:   v4.1- return no errors

Format of message buffer:
Offset  Size    Description
00h    BYTE    reserved
01h    BYTE    message type
00h general
01h server warning
02h-7Fh reserved
80h-FFh user-defined
02h 16 BYTEs   ASCIZ destination machine name
12h 16 BYTEs   ASCIZ server name which user must be logged into
22h 16 BYTEs   ASCIZ user name
32h 16 BYTEs   ASCIZ originating machine name (filled in when received)
42h 80 BYTEs   message text

See also: 215F99

215F98 - LANtastic - SEND UNSOLICITED MESSAGE