homelab-infra/apps/smtp-relay

SMTP relay (Postfix + sasl-xoauth2 → Microsoft 365)

One internal SMTP endpoint that in-cluster apps use for outbound mail. It authenticates to Exchange Online with OAuth2 (XOAUTH2) via mauroreggio/postfix-365, which bundles sasl-xoauth2 — tokens are refreshed inside the SASL layer, no sidecar/cron. Basic-auth SMTP (app passwords) is being retired by Microsoft; this is the modern replacement.

Authelia / Gitea / … ──plain SMTP :25 (in-cluster, no auth)──▶ smtp-relay ──587 STARTTLS + XOAUTH2──▶ smtp.office365.com

Reach it at: smtp-relay.smtp-relay.svc.cluster.local:25. Sends as [email protected].

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Prerequisites (Microsoft 365 / Entra — mostly interactive, one-time)

1. ddupan.top is a verified domain in the tenant (Admin center → Settings → Domains, Status = Healthy). If not, add it and complete the TXT verification (DNS is Cloudflare — can be done in ../../../infrastructure/cloudflared/terraform/).
2. Mailbox [email protected] exists with an Exchange Online license.
3. Authenticated SMTP enabled on it: Admin center → Users → that user → Mail → Manage email apps → tick Authenticated SMTP. (OAuth won't work without this.)

Step 1 — Entra app registration (as code)

cd terraform
az login          # account that can create app regs AND grant admin consent (Global Admin)
terraform init
terraform apply   # creates the app, delegated Graph SMTP.Send + admin consent, a client secret

Grab the three values for the k8s secret:

terraform output -raw client_id
terraform output -raw tenant_id

This is a PUBLIC client (device-code flow) — there is no client secret. Leave CLIENT_SECRET empty in secret.yaml. Presenting a secret makes Entra reject the token refresh with AADSTS700025 "Client is public...".

Step 2 — Deploy the relay

cd ..
cp secret.example.yaml secret.yaml      # paste CLIENT_ID / CLIENT_SECRET / TENANT_ID
kubectl apply -f namespace.yaml
kubectl apply -f secret.yaml -f pvc.yaml
kubectl apply -f deployment.yaml -f service.yaml
kubectl -n smtp-relay rollout status deploy/smtp-relay

At this point Postfix runs but has no token yet, so relaying fails until step 3.

Step 3 — Bootstrap the token (one-time, interactive device-code)

The image's sasl-xoauth2-tool needs the msal Python module, which isn't bundled. Install it ephemerally (only needed for this one mint; the C++ SASL plugin refreshes without it):

POD=$(kubectl get pod -n smtp-relay -l app=smtp-relay -o name | head -1 | cut -d/ -f2)
kubectl exec -n smtp-relay $POD -- sh -c 'python3 -m ensurepip >/dev/null 2>&1; python3 -m pip install -q msal'

Mint the token (env vars come from the pod's secret; CLIENT_SECRET is empty → public flow):

kubectl exec -n smtp-relay -it deploy/smtp-relay -- sh -c \
  'sasl-xoauth2-tool get-token outlook /etc/tokens/[email protected] \
     --client-id="$CLIENT_ID" --tenant="$TENANT_ID" --client-secret="$CLIENT_SECRET" --use-device-flow'

It prints a URL + code — open https://microsoft.com/devicelogin and sign in as the SENDER mailbox [email protected] (NOT yourself/the admin — a token minted for the wrong user gives 535 5.7.3). If prompted for a client secret, press Enter. Then fix ownership so Postfix can read/rewrite it:

kubectl exec -n smtp-relay $POD -- chown postfix:postfix /etc/tokens/[email protected]

Verify identity if unsure: decode the token and check upn == [email protected]. The token persists on the PVC; sasl-xoauth2 refreshes it automatically thereafter.

Step 4 — Test

kubectl exec -n smtp-relay $POD -- sh -c \
  'echo "Subject: relay test\n\nhello" | sendmail -f [email protected] [email protected]'
kubectl exec -n smtp-relay $POD -- tail -n 40 /var/log/maillog   # look for "status=sent"

Step 5 — Point apps at it

• Authelia — replace the filesystem notifier with SMTP in ../authelia/values.yaml: address smtp://smtp-relay.smtp-relay.svc.cluster.local:25, sender [email protected], disable_require_tls: true (plain in-cluster hop). 2FA enrollment codes then go to real email.
• Any future app: same address, From = [email protected] (O365 rejects other senders with 5.7.60 unless a send-as alias is configured in Exchange).

Deliverability (keep mail out of Junk)

• SPF / MX / DMARC for ddupan.top already exist (M365 domain setup).
• DKIM: ✅ enabled 2026-07-28 (Enabled: True, Status: Valid). CNAMEs are in ../../infrastructure/cloudflared/terraform/ (selector1/2._domainkey); signing was turned on with scripts/enable-dkim.ps1 then scripts/enable-dkim-finish.ps1.
  • ⚠️ The CNAME target is NXDOMAIN until signing is enabled. Microsoft creates the tenant host (<tenant>.d-v1.dkim.mail.microsoft) only at enable time, so a correct CNAME looks broken beforehand and Get-DkimSigningConfig reports CnameMissing. Do not go hunting for the "real" CNAME value — run step 2 and re-check DNS.
  • ⚠️ Both scripts deadlock if run via ! or with output redirected to a file — the device code never becomes visible. Run under a PTY: DOTNET_SYSTEM_NET_DISABLEIPV6=1 script -qfc "pwsh -NoProfile -File scripts/enable-dkim.ps1" /tmp/dkim.log The DISABLEIPV6 is required on the laptop — see the IPv6 trap in the root CLAUDE.md; without it Connect-ExchangeOnline hangs in SYN-SENT with no output at all.
• Still soft: SPF is ~all and DMARC is p=none. Harden to -all / p=quarantine once aggregate reports confirm DKIM passes — not before, or you quarantine your own mail.

Notes / gotchas

• PUBLIC client, no secret — see Step 1. CLIENT_SECRET stays empty.
• Sign in as noreply@ (the sender), not the admin, during the Step 3 device login — a token minted for the wrong identity fails with 535 5.7.3.
• msal is ephemeral — reinstall it in the pod (Step 3) before any re-mint; it's gone after a restart but only the one-time mint needs it.
• Token PVC is writable state, not in Git. The refresh token rotates; it lives only on the PVC. Back it up if you want to avoid re-running step 3.
• Refresh-token longevity: Azure AD refresh tokens renew on use but can expire under Conditional Access / long idle — if relaying suddenly fails auth, re-run step 3.