Forked from T1113, step 3.

Initial plan was to achieve this after T1116 - per domain (step 2), but feel free to implement this before, order doesn't matter.

Step 3: user preferences

Requests to server configuration receives the full mail address to the query string. That allows to personalize the message according user preferences.

There are two obvious cases:

• Standards push for STARTTLS, but @xcombelle begs to differ, concerned by downgrade attacks. In such cases, we can allow users to opt in to TLS.
• We prioritize IMAP over POP3. We = both modern mail users agents or our SRV DNS records. Yet, we can allow users to opt in to POP3. But that will ask to convince upstream to extend the configuration format.