Enabling support for the SMTP STARTTLS

This adds support for opportunistic TLS connections for SMTP-only. Clients that follow the RFC of:

  S: 220 inbucket Inbucket SMTP ready
  C: EHLO openssl.client.net
  S: 250-Great, let's get this show on the road
  S: 250-8BITMIME
  S: 250-STARTTLS
  S: 250 SIZE 10240000
  C: STARTTLS
  S: 220 STARTTLS
  C: <starts TLS negotiation>
  C & S: <negotiate a TLS session>
  C & S: <check result of negotiation>
  C: EHLO nowhere.tld
  . . .

This feature is disabled by default.

Setup

To generate x509 certs:

$ openssl req -x509 -sha256 -newkey rsa:2048 -keyout certificate.key -out certificate.crt -days 1024 -nodes

Run inbucket with TLS Enabled (the default private key is cert.key and default public key is cert.crt) -- you can change these with environmental variable also:

$ INBUCKET_SMTP_TLSENABLED=true ./inbucket

To use openssl's s_client for testing:

$ cat << EOF | openssl s_client -tls1_2 -starttls smtp -crlf -connect 127.0.0.1:2500 -ign_eof
ehlo nowhere.tld
mail from: <me@me.me>
rcpt to: <you@you.you>
data
Subject: Hello

This is the body

.
QUIT
EOF

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