message-generate-new-buffers

message-generate-new-buffers is a variable defined in `message.el'.
Its value is

unsent

Documentation:
*Say whether to create a new message buffer to compose a message.
Valid values include:

nil
Generate the buffer name in the Message way (e.g., *mail*, *news*,
*mail to whom*, *news on group*, etc.) and continue editing in the
existing buffer of that name. If there is no such buffer, it will
be newly created.

`unique' or t
Create the new buffer with the name generated in the Message way.

`unsent'
Similar to `unique' but the buffer name begins with "*unsent ".

`standard'
Similar to nil but the buffer name is simpler like *mail message*.

function
If this is a function, call that function with three parameters:
The type, the To address and the group name (any of these may be nil).
The function should return the new buffer name.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.