Egress Gateway Considerations for Mail Delivery

The Problem

Kubernetes uses the node’s IP address as the source IP for outbound connections from pods. In a multi-node cluster, this means your mail server can send emails from different IP addresses depending on which node hosts the pod at any given time.

Why this matters for email:

  • SPF (Sender Policy Framework) requires you to list all authorized sending IPs in DNS

  • Multiple sending IPs means larger SPF records and brittle configuration

  • Adding/removing nodes requires DNS updates

  • Inconsistent sender IPs may hurt email reputation

Solution Options

Option 1: List All Node IPs in SPF

Add every node’s public IP to your SPF record.

When to use:

  • Small, stable cluster (2-3 nodes)

  • Nodes rarely change

  • Simple infrastructure without CNI egress features

Trade-offs:

  • ❌ Brittle - every node change requires DNS update

  • ❌ SPF records have 10 DNS lookup limit

  • ❌ No consistent sender identity

Option 3: LoadBalancer with Static Egress IP

Use a cloud LoadBalancer’s static IP for outbound traffic.

When to use:

  • Cloud provider supports egress via LoadBalancer

  • Need high availability with automatic failover

  • Budget allows for LoadBalancer costs

Trade-offs:

  • ✅ HA with automatic failover

  • ✅ Cloud-native solution

  • ⚠️ Additional cost (cloud LoadBalancer)

  • ⚠️ Limited provider support for egress

IP Stability Choices

When implementing egress gateway, you can use:

Server’s Native IP

The node’s assigned public IP.

When to use: Cost-sensitive deployments, low node turnover

Considerations:

  • Zero additional cost

  • IP changes if node is replaced

  • Requires DNS update when node changes

Floating IP

Cloud provider’s reassignable static IP (e.g., Hetzner Floating IP).

When to use: Need to survive node replacements without DNS changes

Considerations:

  • Additional cost (~€1-2/month)

  • IP survives node replacement

  • Can be reassigned to new node

IPv6 Considerations

Cilium egress gateway (as of v1.17.0):

  • ✅ Full support for IPv4 egress policies

  • ⚠️ Limited IPv6 support - uses node’s native IPv6 address

If dual-stack IPv6 is critical:

  • Include node’s IPv6 address in SPF record

  • Configure reverse DNS (PTR) for IPv6 address

  • Monitor Cilium roadmap for IPv6 egress gateway support

Reverse DNS (PTR Records)

Email servers check that your sending IP resolves to your mail server hostname.

Requirements:

  • PTR record: your-ip → mail.example.com

  • Forward DNS: mail.example.com → your-ip

  • Matches HELO hostname in SMTP

Configuration: Set in cloud provider console (e.g., Hetzner: Servers → Networking → Reverse DNS)

The cdk8s-mailu Library

This library generates Kubernetes manifests for Mailu but does not configure egress routing - that’s a cluster-level networking concern.

What this library provides:

  • Pod manifests for all Mailu components

  • Service definitions for internal routing

  • Configuration for TLS, storage, databases

What you must configure separately:

  • Egress gateway (via Cilium, CNI, or cloud provider)

  • DNS SPF records authorizing sending IPs

  • PTR (reverse DNS) records

  • LoadBalancer or Ingress for inbound traffic

See Also