こちらの設定を参考にさせて頂きました。
https://jedipunkz.github.io/blog/2013/08/12/emacs-mew-gmail/
http://d.hatena.ne.jp/rubikitch/20100502/mail
~/.emacs
(autoload 'mew "mew" nil t)
(autoload 'mew-send "mew" nil t)
(setq mew-fcc "+outbox")
(setq exec-path (cons "/usr/bin" exec-path))
(setq user-mail-address "Your Gmail Address@gmail.com")
(setq user-full-name "Your Name")
(setq mew-smtp-server "smtp.gmail.com")
(require 'mew)
(setq mail-user-agent 'mew-user-agent)
(define-mail-user-agent
'mew-user-agent
'mew-user-agent-compose
'mew-draft-send-message
'mew-draft-kill
'mew-send-hook)
~/.mew.el
; Stunnel
(setq mew-prog-ssl "/usr/bin/stunnel4")
; IMAP for Gmail
(setq mew-proto "%")
(setq mew-imap-server "imap.gmail.com")
(setq mew-imap-user "Your Gmail Address@gmail.com")
(setq mew-imap-auth t)
(setq mew-imap-ssl t)
(setq mew-imap-ssl-port "993")
(setq mew-smtp-auth t)
(setq mew-smtp-ssl t)
(setq mew-smtp-ssl-port "465")
(setq mew-smtp-user "Your Gmail Address@gmail.com")
(setq mew-smtp-server "smtp.gmail.com")
(setq mew-fcc "%Sent")
(setq mew-imap-trash-folder "%[Gmail]/ゴミ箱")
(setq mew-use-cached-passwd t)
(setq mew-ssl-verify-level 0)