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

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. 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 — alert definitions, Uptime Kuma
• Networking Architecture — MetalLB, pfSense NAT, Cloudflare DNS
• Security Architecture — CrowdSec deployment
• Secrets Management — Vault paths for mail credentials
• Mailserver Hardening Plan — historical