Int 15 Fn 05  - Multidos Plus - Check Mailbox                              [T]

   AH = 05h
   AL = mailbox number (00h-3Fh)

Return: AH = status
       00h successful
       DX = length of first message in queue, 0000h if no message
       02h invalid mailbox number

See Also: AH=04h"MultiDOS",AH=06h"MultiDOS"