remote: Fix support for IP literals in recipient addresses

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.**