INT 15 - DESQview 2.50+ - SEND MESSAGE - "READINTO" - GET NEXT MAIL MESSAGE
        AH = 12h
        BH = 13h
        BL = mailbox from which to read
            00h DWORD on top of stack is mailbox handle
            02h use given task's mailbox (task's handle on top of stack)
            03h use current task's default mailbox
        STACK: DWORD size of buffer in bytes
               DWORD pointer to buffer
Return: STACK: DWORD number of bytes read
Program: DESQview 2.5x is distributed as part of DESQview/X 1.0x
Notes:  this call blocks if no input is available, but will return less than
          the requested number of bytes if some (but insufficient) data is
          available
        use this call instead of AH=12h/BH=04h if the mailbox has flag bits
          4 or 5 set, as common memory may be exhausted by that call when
          attempting to read the next message
SeeAlso: AH=12h/BH=05h,AH=12h/BH=16h