ADR 09: Two-tier email

Self-hosted mail for transactional, Listmonk plus SES for scale.

Status: adopted (2026-07-18)

Context

Need: transactional email for the fleet, newsletters for small clients, and one client with a 1.5M-subscriber list wanting cheap, high-quality newsletters. Vendor claim of “80k emails/week for $0 on a $5 VPS” is credible at that volume and dangerously wrong at 1.5M per send.

Decision

Tier 1, fleet transactional and small newsletters: the Openship mail server on our VPS. Per-client domains, DKIM auto-configured, low volume, high value.

Tier 2, the 1.5M list: Listmonk (self-hosted campaign manager, built for multi-million lists) relaying through Amazon SES at $0.10 per thousand. A full-list send costs about $150 versus $10k+/mo for an ESP at that contact count. Dedicated sending subdomain (news.clientdomain.com) so reputation is isolated.

Non-negotiables at scale

Never mix the bulk stream and fleet transactional on the same IP or domain reputation. One bad campaign must not take down password resets for every client.
Validate the list before first send (expect 5 to 10 percent dead addresses; over 5 percent bounces gets the SES account suspended).
Warm up over weeks with ramping volume. SES production access starts around 50k/day and is raised on request.
Wire SES bounce and complaint webhooks into Listmonk suppression.
Gmail/Yahoo bulk rules: DMARC, one-click unsubscribe, spam rate under 0.3 percent in Postmaster Tools.

This is also a sellable service: running a 1.5M newsletter for hundreds per month in infra instead of five figures in ESP fees.