SMTP

Outbound KumoMTA engine that accepts mail from mail and delivers via SMTP (Mailpit locally).

Overview

Property	Value
Directory	`apps/backend/smtp`
Docker service	`smtp` (container `reloop-smtp`)
SMTP ports	`465`, `587`, `2025`, `2465`, `2587`
HTTP port	`8020`
Stack	KumoMTA · Lua

Quick start

bun docker:up

View captured mail at http://localhost:8025 (Mailpit).

Configuration

Key variables in local/docker-compose.yml:

Variable	Default
`BASE_URL`	`https://local.reloop.sh`
`NATS_URL`	`reloop-nats:4222`
`SMTP_HOSTNAME`	`smtp.reloop.sh`

SMTP_HOSTNAME

The hostname senders connect to for submission. It is the relay's own EHLO name, and the dashboard reads it to fill the SMTP → Credentials panel and every code example on that page, so self-hosted users are shown their own endpoint instead of Reloop's.

The installer defaults it to your Reloop domain and preserves it across re-runs. Set it explicitly when submission runs on a different host:

SMTP_HOSTNAME=mail.example.com

It is read at request time, so both services pick up a change on restart:

reloop restart smtp dashboard

Leave it unset and the dashboard falls back to smtp.reloop.sh, which is correct for Reloop Cloud and wrong for every self-hosted install, so set it.

Commands

Command	Description
`bun docker:up`	Start full Docker stack (includes SMTP)
`docker compose -f local/docker-compose.yml up -d smtp`	Start SMTP only
`docker logs -f reloop-smtp`	Stream logs
`docker compose -f local/docker-compose.yml stop smtp`	Stop container

Architecture

Layer	Detail
Submission	Accepts outbound mail on ports `465` / `587` / `2025` / `2465` / `2587`
Policies	Lua scripts in `apps/backend/smtp/policy/` handle routing and DKIM
Local delivery	Captured by Mailpit at http://localhost:8025
Inbound	Port `25` MX is handled by inbound, not this service

Setup

One-command local bootstrap

Port reference

Edge ports and conflicts

Mail

HTTP send API that hands off here

Inbound

MX receiver on port 25