What it does: - Full SMTP server (inbound/outbound, DKIM signing, SPF/DMARC checking) - IMAP with IDLE support - CalDAV/CardDAV (replace Google Calendar/Contacts) - Web admin panel with Prometheus metrics - Greylisting for spam prevention - Auto-discovery (mail clients configure themselves) - Audit logging for compliance

What it doesn't do: - Webmail (use Roundcube, etc.) - ML-based spam filtering (greylisting + basic heuristics only) - Clustering/HA