Currently xendmail uses STARTTLS with no fallback.

I should change it to use SMTP with implicit TLS and only use STARTTLS is the former is not available (and maybe warn about this...?).

A downside is that using STARTTLS will be a bit slower, and my DNS records are only set up for STARTTLS, so that should be fixed too.

That aside, the resolution algorithm needs to distinguish between:

• Some results found (use this)
• No results found (try next protocol)
• Decidedly not available (try next protocol).
• Error in DNS query (???).