[feat] Denylist for outbound mail

It would be useful a denylist for outbound smtp messages to block messages directed to a certain domain or email address.

Start by tracing the outbound SMTP message flow and identify where recipient domains and email addresses are checked before delivery. Clarify how the denylist is configured, whether domain and exact-address rules both apply, and what behavior is expected for blocked messages. Done means the rules are documented, enforced consistently, and covered by tests.

Written by the indexing model from the issue text.