mu4e as Emacs’ default program for sending mail,
see Default email client.
mu4e buries the message buffer after sending; if you want
to kill the buffer instead, add something like the following to your
configuration:
(setq message-kill-buffer-on-exit t)
message-dont-reply-to-names to
mu4e-personal-or-alternative-address-p. In order for this to work
properly you need to pass your address to mu init --my-address= at
database initialization time, and/or use message-alternative-emails.