Gap 1: third shim listener :2526, plaintext, does NOT advertise STARTTLS (TLSConfig
nil) — for opportunistic-STARTTLS clients with no cert-skip (cal.com, nextcloud).
Gap 2: SMTPMapping tls_mode (picks port 2525/2526/2465) + from_domain_var (split
local-part + domain for nextcloud's MAIL_FROM_ADDRESS/MAIL_DOMAIN). Default keeps
existing apps on 2525. Hub untouched.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>