/MSAY begin|verbatim|send|send_to|toggle|toggle_verbatim|abort

Send a multi-line message.  To write a single message with several lines, the 'multi-line mode' should be used.
In multi-line mode, each line (except command lines) typed in the input line will be added to the multi-line message.  Once the message is finished, it can be sent to the current selected buddy with the "/msay send" command.
The 'begin' subcommand enables multi-line mode.  Note that it allows a message subject to be specified.
The 'verbatim' multi-line mode disables commands, so that it is possible to enter lines starting with a slash.  Only the "/msay" command (with send or abort parameters) can be used to exit verbatim mode.
The 'toggle' subcommand can be bound to a key to use the multi-line mode quickly (for example, "bind M13 = msay toggle" to switch using the Meta-Enter combination).

/msay begin [subject]
 Enter multi-line mode
/msay verbatim
 Enter verbatim multi-line mode
/msay send [-n|-h]
 Send the current multi-line message to the currently selected buddy
/msay send_to [-n|-h] jid
 Send the current multi-line message to "jid"
/msay toggle|toggle_verbatim
 Switch to/from multi-line mode (begin/send)
/msay abort
 Leave multi-line mode without sending the message

The -n or -h flags turn the message to "normal" or "headline" accordingly, as opposed to default "chat" message.