infra/docs/architecture/mailserver.md

# Mail Server Architecture

Last updated: 2026-04-18 (SPF switched to Brevo; DMARC reporting address normalized)

## Overview

Self-hosted email for `viktorbarzin.me` using docker-mailserver 15.0.0 on Kubernetes. Inbound mail arrives directly via MX record to the home IP on port 25. Outbound mail relays through Brevo EU (`smtp-relay.brevo.com:587` — migrated from Mailgun on 2026-04-12; SPF record cut over on 2026-04-18). Roundcubemail provides webmail access. CrowdSec protects SMTP/IMAP from brute-force attacks using real client IPs via `externalTrafficPolicy: Local` on a dedicated MetalLB IP.

## Architecture Diagram

```mermaid
graph TB
    subgraph "Inbound Mail"
        SENDER[Sending MTA] -->|MX lookup| MX[mail.viktorbarzin.me:25]
        MX -->|176.12.22.76:25| PF[pfSense NAT]
        PF -->|10.0.20.202:25| MLB[MetalLB<br/>ETP: Local]
        MLB --> POSTFIX[Postfix MTA]
    end

    subgraph "Mail Processing"
        POSTFIX --> RSPAMD[Rspamd<br/>Spam/DKIM/DMARC]
        RSPAMD --> DOVECOT[Dovecot IMAP]
        DOVECOT --> MAILBOX[(Mailboxes<br/>proxmox-lvm PVC)]
    end

    subgraph "Outbound Mail"
        POSTFIX_OUT[Postfix] -->|SASL + TLS| MAILGUN[Brevo EU Relay<br/>smtp-relay.brevo.com:587]
        MAILGUN --> RECIPIENT[Recipient]
    end

    subgraph "Webmail"
        USER[User] -->|HTTPS| TRAEFIK[Traefik Ingress]
        TRAEFIK --> RC[Roundcubemail]
        RC -->|IMAP 993| DOVECOT
        RC -->|SMTP 587| POSTFIX_OUT
    end

    subgraph "Security"
        MLB -->|Real client IPs| CS_AGENT[CrowdSec Agent<br/>postfix + dovecot parsers]
        CS_AGENT --> CS_LAPI[CrowdSec LAPI]
    end

    subgraph "Monitoring"
        PROBE[E2E Roundtrip Probe<br/>CronJob every 20m] -->|Mailgun API| SENDER
        PROBE -->|IMAP check| DOVECOT
        PROBE --> PUSH[Pushgateway + Uptime Kuma]
        DEXP[Dovecot Exporter<br/>:9166] --> PROM[Prometheus]
    end
```

## Components

| Component | Version | Location | Purpose |
|-----------|---------|----------|---------|
| docker-mailserver | 15.0.0 | `mailserver` namespace | Postfix MTA + Dovecot IMAP + Rspamd |
| Roundcubemail | 1.6.13-apache | `mailserver` namespace | Webmail UI (MySQL-backed) |
| Dovecot Exporter | latest | Sidecar in mailserver pod | Prometheus metrics (port 9166) |
| Rspamd | Built into docker-mailserver | — | Spam filtering, DKIM signing, DMARC verification |
| Brevo EU (ex-Sendinblue) | SaaS | — | Outbound SMTP relay (300/day free) |

## Mail Flow

### Inbound
```
Internet → MX: mail.viktorbarzin.me (priority 1)
         → A record: 176.12.22.76 (non-proxied Cloudflare DNS-only)
         → pfSense NAT: port 25 → 10.0.20.202:25
         → MetalLB (dedicated IP, ETP: Local — preserves real client IPs)
         → Postfix → Rspamd (spam + DKIM + DMARC check) → Dovecot → mailbox
```

No backup MX. If the server is down, sender MTAs queue and retry for 4-5 days per SMTP standards (RFC 5321).

### Outbound
```
Postfix → relayhost [smtp-relay.brevo.com]:587 (SASL auth + TLS required)
        → Brevo handles IP reputation, deliverability, bounce processing
        → 300 emails/day free tier (migrated from Mailgun 100/day on 2026-04-12)
```

### Webmail
```
https://mail.viktorbarzin.me → Traefik → Roundcubemail
  IMAP: ssl://mailserver:993 (internal K8s service)
  SMTP: tls://mailserver:587 (internal K8s service)
  DB: MySQL (mysql.dbaas.svc.cluster.local)
```

## DNS Records

All managed in Terraform at `stacks/cloudflared/modules/cloudflared/cloudflare.tf`.

| Type | Name | Value | Purpose |
|------|------|-------|---------|
| MX | `viktorbarzin.me` | `mail.viktorbarzin.me` (pri 1) | Inbound mail routing |
| A | `mail.viktorbarzin.me` | `176.12.22.76` (non-proxied) | Mail server IP |
| AAAA | `mail.viktorbarzin.me` | `2001:470:6e:43d::2` | IPv6 (HE tunnel) |
| TXT (SPF) | `viktorbarzin.me` | `v=spf1 include:spf.brevo.com ~all` | Authorize Brevo for outbound (soft-fail during cutover; was `include:mailgun.org -all` until 2026-04-18 Brevo migration) |
| TXT (DKIM) | `s1._domainkey` | RSA 1024-bit key | Mailgun DKIM (roundtrip probe only — inbound testing still uses Mailgun API) |
| TXT (DKIM) | `mail._domainkey` | RSA 2048-bit key | Rspamd self-hosted DKIM signing |
| CNAME (DKIM) | `brevo1._domainkey` | b1.viktorbarzin-me.dkim.brevo.com | Brevo outbound DKIM (delegated) |
| CNAME (DKIM) | `brevo2._domainkey` | b2.viktorbarzin-me.dkim.brevo.com | Brevo outbound DKIM (delegated) |
| TXT | `viktorbarzin.me` | `brevo-code:a6ef1dd9...` | Brevo domain verification |
| TXT (DMARC) | `_dmarc` | `p=quarantine; pct=100; rua=mailto:dmarc@viktorbarzin.me` | DMARC enforcement; aggregate reports land in-domain at `dmarc@viktorbarzin.me` (tracked under code-569; current live record still points at `e21c0ff8@dmarc.mailgun.org` pending cutover) |
| TXT (MTA-STS) | `_mta-sts` | `v=STSv1; id=20260412` | TLS enforcement for inbound |
| TXT (TLSRPT) | `_smtp._tls` | `v=TLSRPTv1; rua=mailto:postmaster@...` | TLS failure reporting |

### Known Limitation: PTR Mismatch

Reverse DNS for `176.12.22.76` returns `176-12-22-76.pon.spectrumnet.bg.` (ISP-assigned) instead of `mail.viktorbarzin.me`. This is ISP-controlled and cannot be changed on a residential connection. Most modern providers (Gmail, Outlook) rely on SPF/DKIM/DMARC rather than PTR, so impact is minimal.

## Security

### CrowdSec Integration
- **Collections**: `crowdsecurity/postfix` + `crowdsecurity/dovecot` (installed)
- **Log acquisition**: CrowdSec agents parse mailserver pod logs for brute-force patterns
- **Real client IPs**: `externalTrafficPolicy: Local` on dedicated MetalLB IP `10.0.20.202` preserves original client IPs (not SNATed to node IPs)
- **Decisions**: CrowdSec bans/challenges attackers via firewall bouncer rules

### Fail2ban Disabled (CrowdSec is the Policy)

docker-mailserver ships Fail2ban, but it is explicitly disabled here: `ENABLE_FAIL2BAN = "0"` at [`stacks/mailserver/modules/mailserver/main.tf:68`](../../stacks/mailserver/modules/mailserver/main.tf). CrowdSec is the cluster-wide bouncer for SSH, HTTP, and SMTP/IMAP brute-force defence — it already parses the `postfix` and `dovecot` log streams via the collections listed above and applies decisions at the LB/firewall layer. Enabling Fail2ban in-pod would create a duplicate response path (two systems racing to ban the same IP from different enforcement points), add iptables churn inside the container, and fragment the audit trail across two decision stores. Decision (2026-04-18): keep it disabled; CrowdSec owns this policy.

### Rspamd
- Spam filtering with phishing detection and Oletools
- DKIM signing (selector `mail`, 2048-bit RSA)
- DMARC verification on inbound mail
- Auto-learns from Junk folder movements (`RSPAMD_LEARN=1`)
- SRS (Sender Rewriting Scheme) enabled for forwarded mail

### Postfix Rate Limiting
```
smtpd_client_connection_rate_limit = 10  # per minute per client
smtpd_client_message_rate_limit = 30     # per minute per client
anvil_rate_time_unit = 60s
```

### TLS
- Wildcard Let's Encrypt cert (`*.viktorbarzin.me`) for SMTP STARTTLS and IMAPS
- Renewed via Woodpecker CI cron pipeline (DNS-01 challenge via Cloudflare)
- MTA-STS enforces TLS for inbound delivery

## Monitoring

### E2E Roundtrip Probe
CronJob `email-roundtrip-monitor` (every 10 min):
1. Sends test email via Mailgun HTTP API to `smoke-test@viktorbarzin.me`
2. Email hits MX → Postfix → catch-all delivers to `spam@` mailbox
3. Verifies delivery via IMAP (searches by UUID marker)
4. Deletes test email, pushes metrics to Pushgateway + Uptime Kuma

### Prometheus Alerts
| Alert | Threshold | Severity |
|-------|-----------|----------|
| MailServerDown | No replicas for 5m | warning |
| EmailRoundtripFailing | Probe failing for 30m | warning |
| EmailRoundtripStale | No success in >40m | warning |
| EmailRoundtripNeverRun | Metric absent for 40m | warning |

### Uptime Kuma Monitors
- TCP SMTP on `176.12.22.76:25` (external, 60s interval)
- TCP IMAP on `10.0.20.202:993` (internal)
- E2E Push monitor (receives push from roundtrip probe)

### Dovecot Exporter
- Sidecar container in mailserver pod, port 9166
- Scraped by Prometheus for IMAP connection metrics

## Terraform

| Stack | Path | Resources |
|-------|------|-----------|
| Mailserver | `stacks/mailserver/` | Namespace, deployment, service, CronJob, PVCs |
| DNS | `stacks/cloudflared/modules/cloudflared/cloudflare.tf` | MX, SPF, DKIM, DMARC, MTA-STS, TLSRPT records |
| Monitoring | `stacks/monitoring/` | Prometheus alert rules |
| CrowdSec | `stacks/crowdsec/` | Collections, log acquisition (already configured) |

### Secrets (Vault)
| Path | Key | Purpose |
|------|-----|---------|
| `secret/platform` | `mailserver_accounts` | User credentials (JSON) |
| `secret/platform` | `mailserver_aliases` | Postfix virtual aliases |
| `secret/platform` | `mailserver_opendkim_key` | DKIM private key |
| `secret/platform` | `mailserver_sasl_passwd` | Brevo relay credentials (`[smtp-relay.brevo.com]:587 <login>:<key>`) |
| `secret/viktor` | `mailgun_api_key` | Mailgun API for E2E roundtrip probe (retained for inbound delivery testing only; not used for user mail) |
| `secret/viktor` | `brevo_api_key` | Brevo API key (stored for reference) |

## Storage

| PVC | Size | Storage Class | Purpose |
|-----|------|---------------|---------|
| `mailserver-data-proxmox` | 2Gi (auto-resize 5Gi) | proxmox-lvm | Mail data, state, logs |
| `roundcubemail-html-proxmox` | 1Gi | proxmox-lvm | Roundcube web files |
| `roundcubemail-enigma-proxmox` | 1Gi | proxmox-lvm | Roundcube encryption |

## Decisions & Rationale

### No Backup MX
- **Alternatives considered**: ForwardEmail (free relay), Cloudflare Email Routing, Dynu Store/Forward
- **Decision**: Direct MX only. ForwardEmail relay was evaluated (2026-04-12) and abandoned — its anti-spoofing enforcement rejects legitimate forwarded mail regardless of SPF configuration. Cloudflare Email Routing can't store-and-forward (pass-through proxy only). Dynu ($9.99/yr) is a viable future option.
- **Tradeoff**: If server is down, mail delivery relies on sender MTA retry queues (4-5 days standard). No immediate forwarding to a backup address.

### Brevo for Outbound (migrated from Mailgun 2026-04-12)
- **Decision**: All outbound relays through Brevo EU (ex-Sendinblue). 300 emails/day free tier (3x Mailgun's 100/day).
- **Why migrated**: Mailgun's 100/day limit was too tight — the E2E probe uses ~72/day, leaving only 28 for real mail.
- **DKIM**: Brevo uses delegated DKIM via CNAME (`brevo1._domainkey`, `brevo2._domainkey`). Mailgun's `s1._domainkey` retained for the roundtrip probe (still uses Mailgun API for inbound testing).
- **Tradeoff**: Dependency on Brevo SaaS for outbound.

### Rspamd over SpamAssassin/OpenDKIM
- **Decision**: Rspamd replaces both SpamAssassin and OpenDKIM in a single component
- **Tradeoff**: Higher memory usage (~150-200MB) but simpler stack

### Dedicated MetalLB IP for CrowdSec
- **Decision**: Mailserver gets `10.0.20.202` (separate from shared `10.0.20.200`) with `externalTrafficPolicy: Local`
- **Why**: Shared IP with ETP: Cluster SNATs away real client IPs, making CrowdSec detections and Postfix rate limiting useless
- **Tradeoff**: Uses one extra IP from the MetalLB pool. Requires separate pfSense NAT rule.

## Troubleshooting

### Inbound mail not arriving
1. Check MX: `dig MX viktorbarzin.me +short` → should show `mail.viktorbarzin.me`
2. Check port 25: `nc -zw5 mail.viktorbarzin.me 25`
3. Check pfSense NAT rule: port 25 → `10.0.20.202:25`
4. Check Postfix logs: `kubectl logs -n mailserver deploy/mailserver -c docker-mailserver | grep -E 'from=|reject'`
5. Check if CrowdSec is blocking the sender: `kubectl exec -n crowdsec deploy/crowdsec-lapi -- cscli decisions list`

### Outbound mail failing
1. Check Brevo relay: `kubectl logs -n mailserver deploy/mailserver -c docker-mailserver | grep relay` — should show `relay=smtp-relay.brevo.com`
2. Check SASL credentials: `vault kv get -field=mailserver_sasl_passwd secret/platform` — should show `[smtp-relay.brevo.com]:587`
3. Check Brevo dashboard for delivery status
4. SASL auth failure → verify SMTP key (xsmtpsib-...) and login (a7e778001@smtp-brevo.com)

### E2E roundtrip probe failing
1. Check CronJob: `kubectl get cronjob -n mailserver email-roundtrip-monitor`
2. Check job logs: `kubectl logs -n mailserver -l job-name --tail=20`
3. Check Mailgun rate limit (HTTP 429 errors mean too many API calls)
4. Check IMAP login: verify `spam@viktorbarzin.me` password in Vault (`secret/platform` → `mailserver_accounts`)

### Spam/brute-force attacks
1. Check CrowdSec decisions: `kubectl exec -n crowdsec deploy/crowdsec-lapi -- cscli decisions list`
2. Check Postfix logs for auth failures: `kubectl logs -n mailserver deploy/mailserver -c docker-mailserver | grep 'authentication failed'`
3. Verify real client IPs in logs (not 10.0.20.x node IPs)

## Related

- [Monitoring Architecture](monitoring.md) — alert definitions, Uptime Kuma
- [Networking Architecture](networking.md) — MetalLB, pfSense NAT, Cloudflare DNS
- [Security Architecture](security.md) — CrowdSec deployment
- [Secrets Management](secrets.md) — Vault paths for mail credentials
- [Mailserver Hardening Plan](../plans/2026-02-23-mailserver-hardening-plan.md) — historical