[feat] Reject emails from newly registered domains

In the last days i note increasing of spam using new registered domains.
It could be good to block them at smtp level ASAP.

There are a couple of RBLs to identify new domains, may be one can also share an rsync file or accomodate free dns lookups (Spameatingmonkey Fresh).

The issue names no files, tests, or entry points. Start by locating the SMTP-level rejection path and evaluating the suggested RBLs, then define the data source and behavior; done should include a clear implementation scope and tests showing newly registered domains are rejected.

Written by the indexing model from the issue text.