Feature request - email - starttls support

[r0one]

(I would have gladly opened this issue on the gitea instance, but it was down at the moment I checked. Please tell me if you'd prefer me to close it here and reopen it on gitea!)

Is your feature request related to a problem? Please describe.
I cannot plug plume on my mail relay, which has a typical configuration: submission on port 587, with starttls.

Describe the solution you'd like
Implement a new configuration environment variable, and use the object described here. That would add probably 5-10 new loc in mail.rs.

Describe alternatives you've considered
Spin off a local mail relay, that would listen unencrypted on port 25 and relay emails to my provider on port 587. But that would be a ton of new binaries and processes to monitor but solving just one little issue that lettre::transport::smtp handles seamlessly.

[r0one]

suggested labels: Backend and Feature request.

I do not believe this is "instance specific" - nowadays sending an email is a pain, so it's very common to separate the infrastructure for sending emails, and not to be able to trust the client (because we do not listen on the loopback).