server-kill-new-buffers

server-kill-new-buffers is a variable defined in `server.el'.
Its value is

t

Documentation:
Whether to kill buffers when done with them.
If non-nil, kill a buffer unless it already existed before editing
it with the Emacs server. If nil, kill only buffers as specified by
`server-temp-file-regexp'.
Please note that only buffers that still have a client are killed,
i.e. buffers visited with "emacsclient --no-wait" are never killed
in this way.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.