INT 15 - TopView - SEND MESSAGE - "SUBFROM" - SEND MAILBOX MESSAGE/STAT BY REF
AH = 12h
BH = 0Bh
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:  only a pointer to the message is stored, but the write may still fail
due to insufficient memory
under DV 2.2+, failed mailbox writes may return CF set (see AX=DE15h)

See also: 1512

1512 - TopView - SEND MESSAGE - "SUBFROM" - SEND MAILBOX MESSAGE/STAT BY REF