Allow SMTP over unix socket (& rework mailer settings)

[clarfonthey]

Motivation

Essentially, systems like OpenSMTPD allow listening for SMTP connections on a unix socket directly instead of over localhost. This allows the connections to be faster (unix domain sockets have less overhead than local TCP) and also lets you restrict who can access the socket.

It would be nice if we could provide a path for the mailer.HOST setting, so that it will submit via a unix socket connection instead of TCP. If we were to avoid reworking the mailer settings, we could simply accept a path here, but IMHO we should rework the mailer settings to match the existing server settings, since it would get rid of a lot of weird edge cases like TLS over unix socket.

Proposed settings

The relevant settings right now:

• HOST: accepts something of form addr:port or just addr to imply port (addr can be domain or IP)
• DISABLE_HELO, HELO_HOSTNAME: describe host used in HELO operation; has some weird settings
• SKIP_VERIFY: whether to skip verifying server certificate
• USE_CERTIFICATE, CERT_FILE, KEY_FILE: client certificates
• IS_TLS_ENABLED: confusingly, doesn't determine whether TLS is enabled, but the usage of STARTTLS or plain TLS
• MAILER_TYPE: set to SMTP to use the SMTP mailer

My proposal:

• PROTOCOL: only has effect for SMTP mailer. can be smtp, smtps, smtp+startls, smtp+unix
• 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: scarier version of SKIP_VERIFY
• CLIENT_CERT_FILE, CLIENT_KEY_FILE, USE_CLIENT_CERT: clarify client certificates here
• MAILER_TYPE: still allows smtp, dummy, sendmail

Note: we don't have to do all these renames, but I strongly encourage replacing HOST + IS_TLS_ENABLED with PROTOCOL, SMTP_ADDR, and SMTP_PORT.

After looking at the code, it seems like these changes would be relatively easy to make, and I'm willing to provide those, but I figured that making an issue to discuss this first would be a good idea.

Alternatives

Easy option: just keep the settings as-is and make HOST = /path work for unix sockets. IS_TLS_ENABLED = true will error for unix sockets.

Compromise option: just deprecate HOST and IS_TLS_ENABLED and replace them with PROTOCOL, SMTP_ADDR, and SMTP_PORT. Keep the other options as-is.

Other notes

Having PROTOCOL = smtp should probably be highly not recommended unless SMTP_ADDR = localhost. Not sure where to fit this in the docs for these. I do know that it has some recommendations about only using IS_TLS_ENABLED = false when HOST = localhost.

One big reason why I propose changing all of these settings at once is that every option except modifying HOST to accept paths will break people's configs anyway, and we might as well clean things up while we do that.