INT 15 - TopView - SEND MESSAGE - "ADDTO" - SEND MAILBOX MESSAGE/STAT BY VALUE
AH = 12h
BH = 0Ah
BL = mailbox to write to
00h handle is DWORD on top of stack
02h default mailbox of task whose handle is on top of stack
03h current task's default mailbox
STACK:  DWORD   status (low byte)
DWORD   length of message
DWORD   address of message
Notes:  the message is copied into either system or common memory
insufficient memory normally causes the process to be aborted; under
DESQview 2.2+, failed writes may return CF set instead (see AX=DE15h)

See also: 1512

1512 - TopView - SEND MESSAGE - "ADDTO" - SEND MAILBOX MESSAGE/STAT BY VALUE