remote: Fix support for IP literals in recipient addresses

[foxcpp]

Since a lot of authentication mechanisms are keyed on the MX or domain name, the IP literals support uses the rDNS domain of the IP address. This assumes forward-confirmed rDNS, but it should be fine for most mail servers.

There is, however, an issue with current implementation: it assumes the returned rDNS name is a recipient domain, not a MX what is not true for many mail servers. The correct behavior is to construct MX candidates list from rDNS names.

The new implementation of "security policies" broke it altogether and it is disabled now.

[foxcpp]

Removing from 0.1 milestone. Despite being a standard (RFC 5321), many services and email stack implementations do not support it. I do not see a practical reason to bother bending remote module implementation once more to make it work now.

Current behavior is:

• It is not possible to receive messages with address domain-part using an address literal since destination pipeline directive rejects invalid domains.
• It is not possible to send outbound messages using address literals. 550 5.1.1 IP address literals are not supported error will be returned.

If somebody wants to make it, I would do the following:

• Make most "security policy" implementations (see security.go) no-op if "domain" or "mx" is an address literal.
• Make lookupMX routine return a single MX candidate if "domain" is an address literal.
• Remove existing but disabled code for rDNS-resolution of IP literals.

[yunginnanet]

I think this is the first time I've ever seen someone give themselves the "good first issue" tag on their own repo

jokes aside thanks for all your work. luckily I don't think most people desire to use IP literals in their mx stack.