selfpost/docs/product.md

SelfPost — product boundaries

What this file is. Stable product definition for SelfPost v1.0: purpose, deployment assumptions, explicit out-of-scope items, and the multi-domain model. User-facing overview in README.md; install and operations in guide.md; as-built technical detail in architecture.md.

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Purpose

SelfPost is a self-hosted outbound SMTP relay with a web control panel, shipped as a single Docker image. It sends mail directly to the internet from your own IP with per-domain DKIM signing.

Primary workflow: configure the relay once (domains, DKIM, SASL applications), then point scripts and apps at the SMTP endpoint. SelfPost delivers as the configured sending domain.

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Deployment context (fixed assumptions)

These constraints are intentional; changing them requires an explicit product decision:

1. VPS or home server — one image for both. Raspberry Pi and similar boards are not a current target but not ruled out forever.
2. Send from your own IP (DIY) — no intermediate relay provider. Requires outbound port 25, static IP, and configurable PTR/rDNS from the operator.
3. Single container — Postfix, OpenDKIM, and the panel run under one supervisord inside one image.
4. Panel is internet-facing — mandatory security requirements in security.md.

Infrastructure the operator provides (not SelfPost features): unblocked outbound TCP 25, static IP, PTR/rDNS, acceptable IP reputation. SelfPost does not detect, bypass, or compensate for missing prerequisites; mail simply fails to deliver when they are absent.

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Out of scope

Explicitly excluded to prevent scope creep:

• Inbound mail (IMAP/POP3, mailboxes, delivery to user inboxes)
• Webmail
• Multi-user panel / organisations / roles — one administrator; managing multiple sending domains is in scope (see below)
• Inbound antispam/antivirus (rspamd, ClamAV, etc.)
• A custom MTA — Postfix is used as-is
• Dovecot or a full mail stack for SASL — Cyrus SASL (sasldb2) only

Future line 2.x.x (optional inbound relay, domain-admin role) is tracked in roadmap.md and requires explicit approval before implementation.

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Multi-domain model

SelfPost is a multi-domain outbound relay. Two linked entities:

Sending domain

Example: example.com. Has its own DKIM key and selector.

Application (account)

A SASL login/password bound to one domain. A domain may have several applications (e.g. newsletter vs alerts), each with its own credentials, but each may send only from its own domain, never from another.

From-address mode (per application)

Set when creating or editing an application:

1. Any address in the domain — *@example.com (wildcard in smtpd_sender_login_maps).
2. Explicit address list — only listed From addresses within the domain; anything else is rejected even if it belongs to the same domain.

In both modes the From address must belong to the application's domain.

What the panel manages per domain

• DKIM key + selector (shared by all applications on that domain)
• One or more applications (SASL credentials + From mode + optional rate limits)
• DNS guidance (DKIM TXT; reminders for SPF/DMARC)

Adding a domain does not create an application automatically.

Lifecycle

• Add domain — record, DKIM key, DNS instructions; no application yet.
• Add application — SASL pair (password shown once), From mode, map entries, postfix reload.
• Delete domain — removes DKIM key and all its applications.
• Delete application — removes only that app's SASL and map entries.

This is not multi-tenancy (one admin); it is one owner operating several sending domains with independent application credentials.