Merge mail-edge-hardening: live v0.16 docs, IPv6 egress, outbound-relay lessons, CLAUDE IPv6 fix

The sidecar gained real IPv6 egress (commit 34422ba / LESSONS.md 9), but the
outbound pitfall still asserted 'no IPv6 / no AAAA->A fallback'. Reword to
reflect the fix while keeping the tailnet-relay guidance.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -68,6 +68,11 @@ STALWART_S3_BUCKET=stalwart-mail
 # ----------------------------------------------------------------------------
 # Most VPS providers block outbound :25. If yours does, relay through a
 # smarthost (host:port). Leave blank to attempt direct MX delivery.
+#
+# IMPORTANT: Use an IPv4 literal or a tailnet IP — never a dual-stack hostname.
+# The container has no IPv6 and will NOT fall back from AAAA to A; any host
+# with an AAAA record will fail immediately (os error 101). Relaying over the
+# tailnet (100.x:587) sidesteps this entirely and also bypasses VPS SMTP blocks.
 STALWART_SMARTHOST=
 
 # ----------------------------------------------------------------------------

CLAUDE.md

@@ -81,7 +81,12 @@ healthcheck, ephemeral OAuth auth). Don't drift it. Tag: `tag:stalwart`.
   re-init. And never test a password over `127.0.0.1` against these Postgres
   containers: pg_hba `trust`s loopback and accepts ANY password. Test over the
   tailnet (scram) or you'll fool yourself.
-- **Outbound :25 is usually blocked on VPS.** Set `STALWART_SMARTHOST`.
+- **Outbound :25 is usually blocked on VPS.** Set `STALWART_SMARTHOST`, and
+  prefer relaying over the tailnet (`100.x:587`) — it bypasses the VPS SMTP-port
+  blocks and, having no AAAA, sidesteps the v6-first trap. The sidecar now has
+  its **own IPv6 egress** (LESSONS.md 9), so dual-stack targets resolve too;
+  before that fix an AAAA-only path would hang (`os error 101`) with no fallback
+  to A. See LESSONS.md 8–9.
 - **Mail forces WAN ports.** `:25` must be world-reachable for inbound
   federation — this is the one place the tailnet-only model can't hold. Keep
   submission/IMAP tailnet-only if you want a tighter surface.

LESSONS.md

@@ -94,3 +94,155 @@ duplicate-cert limit, use **certspotter** instead:
 Also: LE limits are dimensioned — **failed validations** are hourly (5/hr/host,
 the one a retry storm trips), **issued duplicates** are weekly (5/wk). A renewal
 task hammering every 10 min trips the hourly one; consolidate to a single task.
+
+## 8. The Stalwart container has no IPv6 — AAAA targets fail before IPv4 is tried
+
+**Symptom:** Outbound delivery (and relay-to-smarthost) to any host with an
+AAAA record fails with `I/O error: Network is unreachable (os error 101)`.
+Hosts that are IPv4-only deliver fine. Pointing a relay at a *hostname* that
+has both A and AAAA fails; pointing it at the raw IPv4 works.
+
+**Cause:** Stalwart shares the `ts-stalwart` sidecar's netns, which has no
+global IPv6. When it resolves a dual-stack target it tries the AAAA first,
+gets `ENETUNREACH` immediately, and for a **relay next-hop it does not fall
+back to the A record** — it just records the v6 failure and backs off. So a
+single missing address family wedges all mail to dual-stack destinations.
+
+**Fix:** Either (a) pin the relay/smarthost `address` to an **IPv4 literal**
+(no AAAA to trip on), or (b) give the container real IPv6. Note that relaying
+over the **tailnet** sidesteps this entirely — you connect to a tailnet
+`100.x` address, which has no AAAA, so the v6-first trap never triggers.
+
+> **RESOLVED (2026-06-11) — option (b) is now done.** The container has real
+> IPv6 egress; this trap no longer fires. See Lesson 9's fix for how.
+
+## 9. Configuring IPv6 on the KVM host does NOT give the container IPv6
+
+**Symptom:** `ip -6 addr` and `ping6 google.com` succeed on the KVM host, but
+Stalwart still dies with `os error 101` on AAAA targets, and the box is still
+a broken IPv6 Tailscale exit node.
+
+**Cause:** The host's `eth0` and the container/sidecar netns are separate
+network stacks. Adding the provider's `/64` to `eth0` (ifupdown `inet6 static`
++ `onlink` default route, since the gateway is in a different /64) fixes the
+*host*, not the container. Docker doesn't hand IPv6 to containers by default,
+and the sidecar routes via Tailscale, not eth0.
+
+**Fix:** Don't assume host IPv6 = container IPv6. Test from *inside* the
+container's netns. For mail egress, the IPv4-literal relay (Lesson 8) or the
+tailnet relay avoids needing container IPv6 at all. Enabling true container
+IPv6 (Docker IPv6 + routing the /64 in) is a separate, larger task.
+
+**RESOLVED (2026-06-11) — the easy way, no /64 routing or ndppd.** Because the
+container only needs IPv6 **egress** (inbound arrives via the edge/tailnet,
+never v6), you don't need a routable prefix or NDP proxy at all — just a **ULA
+subnet + masquerade**, exactly like Docker does for v4:
+```yaml
+# docker-compose.yml
+networks:
+  default:
+    enable_ipv6: true
+    ipam:
+      config:
+        - subnet: fd00:7a17:600d::/64
+          gateway: fd00:7a17:600d::1
+```
+Docker 29 enables `ip6tables` by default and masquerades the ULA out the host's
+global v6, so the sidecar netns (shared by Stalwart via `network_mode`) gets a
+working v6 default route with **zero host sysctl/daemon changes** (host
+`net.ipv6.conf.all.forwarding` was already 1 from the static-v6 setup). Verify
+from *inside* the netns: `ping6 google.com` + a TCP connect to a v6 literal on
+:443. Recreating the network (`docker compose down && up`) bounces the stack and
+the ephemeral sidecar gets a new tailnet IP — MagicDNS covers it (Lesson 6), and
+the MTA route table rebuilds anyway (Lesson 12). This does **not** give inbound
+v6; for that you'd still publish AAAA + make the edge listen on v6 (separate).
+
+## 10. The VPS blocks ALL outbound SMTP ports — relay over the tailnet
+
+**Symptom:** Direct MX delivery and relay-to-public-host both fail with
+`Connection timed out (os error 110)`, and the SYN never arrives at the
+destination. Not just port 25 — `465`, `587`, even alt-port `2525` all time out.
+
+**Cause:** The KVM provider blocks all outbound SMTP submission ports to prevent
+spam. Only non-SMTP ports (`443`, etc.) egress. Confirmed with:
+```bash
+for p in 25 465 587 2525 443; do
+  timeout 5 bash -c "exec 3<>/dev/tcp/<dst>/$p" && echo "$p OPEN" || echo "$p blocked"
+done
+# 443 OPEN, all SMTP ports timeout
+```
+
+**Fix:** Relay over the **tailnet**. Tailscale rides WireGuard/DERP (UDP 41641 /
+443), so it's immune to SMTP port filtering. Point the relay at the smarthost's
+**tailnet IP** (e.g. `100.x:587`), not its public address. Long-term: ask the
+provider to unblock outbound 25/587 for verified use.
+
+## 11. The sidecar can RECEIVE on the tailnet but can't INITIATE without an ACL grant
+
+**Symptom:** The relay to `<mailbox-tailnet-ip>:587` times out (`os error 110`),
+yet the **KVM host** (same physical machine) can reach that exact IP:port over
+the tailnet fine. Looks like a routing or transparent-proxy bug.
+
+**Cause:** The Stalwart container rides the `ts-stalwart` sidecar — a **separate
+tailnet node** (`tag:stalwart`) from the KVM host. The `tailwart` ACL block only
+listed `tag:stalwart` as a **destination** (`"dst": ["tag:stalwart"]`). Tailnet
+is default-deny, so the sidecar could receive connections but could not
+*initiate* the relay back to the mailbox → silent drop → timeout. The KVM host
+worked because it's a different, permitted identity, which masked the real cause.
+
+**Fix:** Add an ACL rule granting `tag:stalwart` as a **source**:
+```json
+{ "src": ["tag:stalwart"], "dst": ["tag:mail"], "ip": ["tcp:587"] }
+```
+(mailbox is `tag:mail`). Applies in seconds, no restart. See `acl-snippet.hujson`.
+
+## 12. Stalwart only rebuilds its MTA route table at container startup
+
+**Symptom:** You edit an `MtaRoute` (address, etc.) via API/UI, but delivery keeps
+using the old value. The datastore shows the new value; live delivery ignores it.
+
+**Cause:** The `routing_strategy` map is built once when the process boots. The
+`ReloadSettings` action reloads the datastore but does **not** rebuild the SMTP
+route map. So route/strategy changes are invisible until restart.
+
+**Fix:** After any `MtaRoute` / `MtaOutboundStrategy` change,
+`docker restart tailwart-stalwart-1`. (Side effect: the ephemeral sidecar gets a
+new tailnet IP each restart — anything addressing it by IP must rediscover it;
+use the MagicDNS name where possible.)
+
+## 13. "Did Stalwart eat my custom DNS records?" — no; Spaceship is RRSet-upsert
+
+**Symptom:** A manually-added record (e.g. an `AAAA` for the apex/`mail`) is
+gone from the zone, and the suspicion is that Stalwart's ACME DNS-01 integration
+overwrote it on a renewal.
+
+**Cause:** Almost never Stalwart. Its **only** DNS-provider writes are
+`_acme-challenge.<name>` TXT (the rotating challenge) and `_validation-persist`
+TXT (the LE account-pinned persistent-validation record). It does **not** create
+or modify A/AAAA/MX/SRV — those you add yourself from its "recommended records"
+page. And the Spaceship API is **RRSet-upsert keyed by (name, type)**, not a
+whole-zone replace: a `PUT /api/v1/dns/records/{domain}` with
+`{"force":true,"items":[…]}` only touches the RRSets named in `items`. Proof:
+25 unrelated records coexist untouched through every rotating `_acme-challenge`
+write; and adding one apex `AAAA` left the other 25 exactly intact (25→26).
+
+So a vanished AAAA is far more likely a **provider-side loss/rollback** (e.g.
+during a data-center DDoS) or a manual edit — not Stalwart.
+
+**How to inspect / verify (read-only), creds in `.env`:**
+```bash
+KEY=$(grep '^SPACESHIP_KEY=' .env | cut -d= -f2)
+SECRET=$(grep '^SPACESHIP_SECRET=' .env | cut -d= -f2)
+curl -s "https://spaceship.dev/api/v1/dns/records/<domain>?take=100&skip=0" \
+  -H "X-Api-Key: $KEY" -H "X-Api-Secret: $SECRET" | python3 -m json.tool
+```
+To add a record, `PUT` the same endpoint with a single-item `items` array — it
+won't disturb siblings. **Snapshot the zone (GET) before any write** and diff
+after; snapshots land in `_backup/` (gitignored). Always re-check at the
+authoritative NS (`dig +short AAAA <name> @launch1.spaceship.net`), not a cache.
+
+**Caveat — don't publish `mail` AAAA before the edge listens on v6.** Inbound
+mail follows `MX → mail.<domain>`; an `AAAA` there with no v6 `:25` listener on
+the edge makes senders try v6 and some won't fall back → deferred/bounced mail.
+An **apex** `AAAA` is safe (it doesn't affect MX routing). Do `mail` AAAA + edge
+v6 listeners together.

README.md

@@ -8,17 +8,20 @@ mail server wired into **Postgres + Redis + Garage S3** at once, deployed as a
 Tailscale sidecar, with a separate `caddy-l4` edge that pipes the raw mail ports
 over the tailnet. For `infinidim.net`.
 
-See [CLAUDE.md](./CLAUDE.md) for the architecture and the gotchas.
+See [CLAUDE.md](./CLAUDE.md) for the architecture and the gotchas, and
+[LESSONS.md](./LESSONS.md) for the hard-won symptom→cause→fix notes from
+bringing it up (PG races, ACME DNS-01, PROXY-protocol trust, IPv6 egress, …).
 
 ## Layout
 
 ```
 tailwart/
-├── docker-compose.yml      # the mailbox: ts-stalwart sidecar + stalwart
+├── docker-compose.yml      # the mailbox: ts-stalwart sidecar + stalwart (+ IPv6 egress)
-├── config/config.toml      # Stalwart config — PG + Redis + S3 wiring (strawman)
+├── config/config.json      # Stalwart v0.16 bootstrap config — datastore (Postgres) ONLY
+├── config/config.toml      # dead v0.15-era config, kept as historical reference
 ├── caddy/                  # the edge: custom Caddy (caddy-l4) layer-4 mail proxy
 │   ├── Dockerfile          #   pulls prebuilt caddy-l4 binary (caddyserver.com, no local build)
-│   ├── caddy.json          #   :25/465/587/143/993 → stalwart over the tailnet
+│   ├── caddy.json          #   :25/465/587/143/993 mail + :443 SNI fan-out → stalwart over the tailnet
 │   ├── docker-compose.yml  #   deploy on any public-IP, tailnet, tag:reverse-proxy host
 │   └── README.md
 ├── acl-snippet.hujson      # tag:stalwart owner + grants to merge into your policy
@@ -26,6 +29,11 @@ tailwart/
 └── .gitignore
 ```
 
+> **v0.16 config model:** `config.json` describes *only* where Postgres lives;
+> everything else (domains, accounts, listeners, ACME, blob/Redis wiring, proxy
+> trust, DKIM, spam) lives **in Postgres**, managed via the web UI or JMAP. The
+> old TOML + `%{env}%` macro model is gone — see CLAUDE.md / LESSONS.md.
+
 ## Quickstart
 
 ```bash
@@ -45,6 +53,12 @@ configuration in Stalwart's web admin (`mail.infinidim.net`).
 
 ## Status
 
-Scaffold / strawman. The Stalwart `config.toml` keys need verifying against a
+Live. Pinned to `stalwartlabs/stalwart:v0.16.7`, booting cleanly against the
-pinned image version before first real boot — treat it as a starting shape, not
+shared Postgres/Redis/Garage backends, holding a Let's Encrypt wildcard via
-a turnkey config.
+ACME DNS-01 (Spaceship), and relaying outbound over the tailnet (the VPS blocks
+all outbound SMTP ports). The container has its own IPv6 egress.
+
+Not yet exposed for inbound IPv6 mail: `mail.infinidim.net` is intentionally
+A-only until the edge proxy gains v6 `:25` listeners — publish its `AAAA` and
+the v6 listeners together, or senders will try v6 and some won't fall back. See
+LESSONS.md.

acl-snippet.hujson

@@ -21,6 +21,12 @@
   "ip":  ["tcp:25", "tcp:465", "tcp:587", "tcp:143", "tcp:993", "tcp:443", "tcp:8080"],
 },
 
-// 4) admin console (not this file): assign tag:stalwart to the same OAuth
+// 4) grant — Stalwart initiates outbound relay to the smarthost (tag:mail).
+//    Without this, the sidecar can RECEIVE but cannot INITIATE over the tailnet
+//    (default-deny; the KVM host is a different identity and won't mask this).
+//    Adjust dst/port to match your smarthost's tag and submission port.
+{ "src": ["tag:stalwart"], "dst": ["tag:mail"], "ip": ["tcp:587"] },
+
+// 5) admin console (not this file): assign tag:stalwart to the same OAuth
 //    client federatedSocial uses, on the Devices/Core + Keys/AuthKeys scopes.
 //    Missing → 403 "calling actor does not have enough permissions" at boot.

docker-compose.yml

@@ -80,3 +80,19 @@ services:
 
 volumes:
   stalwart-data:
+
+# The sidecar's bridge (shared by stalwart via network_mode) gets IPv6 here so
+# the container can reach AAAA-only / dual-stack hosts. Without it the netns has
+# no global v6 → Stalwart tries AAAA first, gets ENETUNREACH, and for a relay
+# next-hop never falls back to A (see LESSONS.md #8). A ULA subnet is fine: we
+# only need *egress* (inbound arrives via the edge/tailnet, never v6). Docker 29
+# masquerades it out the host's global v6 via ip6tables — no routable prefix,
+# NDP proxy, or host sysctl needed. Recreating this network bounces the stack
+# and the ephemeral sidecar gets a new tailnet IP (MagicDNS handles it).
+networks:
+  default:
+    enable_ipv6: true
+    ipam:
+      config:
+        - subnet: fd00:7a17:600d::/64
+          gateway: fd00:7a17:600d::1