priority {cm:2026-03-26} received headers (DONE) {cm:2026-03-26} autocert {cm:2026-03-26} user-agent header (DONE) {cm:2026-03-27} importance header (DONE) {cm:2026-03-27} x-originating-ip (DONE) {cm:2026-03-27} organization header resent-from resent-to resent-date list-id list-help list-unsubscribe list-subscribe list-post list-owner list-archive thread-index thread-topic arc-seal arc-message-signature arc-authentication-results (DONE - Full ARC with RSA+Ed25519) {cm:2026-03-27} ARC cryptographic verification (DONE - Full DKIM-style signature verification with RSA+Ed25519) {cm:2026-04-06} configurable strict-TLS mode (DONE - per-domain require_tls flag with RFC 8689 semantics) {cm:2026-04-06} smtp authentication (plain, login, cram-md5) bounce handling ndr detection spam filtering vacation auto-reply mailing lists message search tag label system message templates backup restore pgp gpg support caldav calendar carddav contacts dane tlsa validation rate limiting ui two-factor authentication monitoring metrics prometheus webhook api mobile app message context threading webmail interface DSN (Delivery Status Notifications) - No detailed bounce reports SMTPUTF8 - No Unicode email support (e.g., 用户@example.com) List-Unsubscribe headers - No automatic unsubscribe links IPv6 - Only IPv4 connections (1-line fix: change "tcp4" to "tcp") VERP - Per-recipient bounce tracking ARF - Abuse report processing DANE, BIMI, Greylisting, Tarpitting, enhanced status codes