* `PROTOCOL`: can be smtp, smtps, smtp+startls, smtp+unix, sendmail, dummy
* `SMTP_ADDR`: domain for SMTP, or path to unix socket
* `SMTP_PORT`: port for SMTP; defaults to 25 for `smtp`, 465 for `smtps`, and 587 for `smtp+startls`
* `ENABLE_HELO`, `HELO_HOSTNAME`: reverse `DISABLE_HELO` to `ENABLE_HELO`; default to false + system hostname
* `FORCE_TRUST_SERVER_CERT`: replace the unclear `SKIP_VERIFY`
* `CLIENT_CERT_FILE`, `CLIENT_KEY_FILE`, `USE_CLIENT_CERT`: clarify client certificates here
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
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