错误提示：
smtpmail-send-it: Sending failed: 530 5.7.0 Must issue a STARTTLS
command first - gsmtp
解决方法：
1、apt install gnutls-bin:
2、;; send mail
(setq
send-mail-function 'smtpmail-send-it
message-send-mail-function 'smtpmail-send-it
user-mail-address "youremail@gmail.com"
smtpmail-starttls-credentials '(("smtp.gmail.com" "587" nil nil))
smtpmail-auth-credentials (expand-file-name "~/.authinfo")
smtpmail-default-smtp-server "smtp.gmail.com"
smtpmail-smtp-server "smtp.gmail.com"
smtpmail-smtp-service 587
smtpmail-debug-info t
starttls-extra-arguments nil
starttls-gnutls-program "/usr/bin/gnutls-cli"
starttls-extra-arguments nil
starttls-use-gnutls t
)