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feat(email): add optional Linux-user SMTP intake on the message bus (#231) - #929
Closes#231. Companion defangdevs/local-channels#107 has merged with green post-merge CI. Its reviewed immutable commit is pinned here.
When enabled, mail to <linux-user>@<configured-hostname> reaches that user's existing message bus. OpenSMTPD handles SMTP/STARTTLS and durable transport retries; a shared MIME adapter runs as the recipient uid and publishes email.received. Normal subscriptions, standing watches, profiles, ownership and admission handle the agent routing.
Intake is disabled by default on both NixOS and native hosts. Only explicitly enabled Linux users and exact configured domains are accepted. An explicit userbase bypasses .forward; envelope data never becomes an MDA command. The adapter preserves MIME and decoded alternatives/attachments in the companion's private retained payload store, using stable IDs and byte-identical JSON on retry. Mail headers and claimed senders remain untrusted; this does not authenticate email authors or add outbound replies.
OpenSMTPD was chosen for its compact declarative fit to this reception-to-local-command path. Postfix could implement the same boundary; no performance/security superiority is claimed. Operators supply root-managed certificate/key files and restart intake after renewal. Existing web/webhook capability is required. Configuration and operational details are in docs/email.md.
Networking, migration and bounds
EC2/Lightsail EmailIngress and Azure emailIngress optionally open inbound TCP 25 on existing cloud firewalls (default false), independently of browser client CIDRs. Outbound SMTP restrictions stay in place. No new cloud resources or IAM grants are added. Enabling assumes port 25 is available; it does not replace an existing MTA or export Caddy's private key.
Incoming MIME defaults to 2 MiB, with 100 MIME parts and per-connection transaction/recipient limits. Retained content is bounded separately (24 hours, 128 MiB, 2,000 records by default); general bus/MTA spool byte quotas remain the separate local-channels#105 work. Disabling stops intake and preserves transport queue files; accepted bus work follows normal routing/expiry.
Ten MIME/adapter/native-email regression tests, including first-boot discovery, runtime refresh, passwd-home agreement and permanent/temporary exit status.
Assembler regression tests, backend parity, and Nix evaluation checks for valid/invalid email domains and Linux users.
AWS CloudFormation lint; Azure Bicep rebuilt and rendered-bootstrap/template checks passed.
Generated module, native config/fixtures and golden fixtures regenerated and reviewed.
Correctness/security review: recipient uid boundary, no relay action, fixed root-owned configuration and executable links, no envelope-to-shell interpolation, no .forward override, private retained content and untrusted sender semantics.
Full native Nix check set after review fixes passed locally and in GitHub CI.
Isolated rootless-container smoke (with the service socket-family sandbox applied): actual source initialization, SMTP/STARTTLS, exact recipient/relay refusals, two messages sharing Message-ID, bus downtime with durable MTA retry, full retained content and normal peer spool routing.
New real SMTP/STARTTLS VM integration: envelope/Bcc-style routing, disabled/unknown/domain/plus/relay refusals, .forward bypass, bus downtime with MTA retry, per-user privacy and complete retained content after restart (passed in GitHub CI on e50f4a2; this host has no KVM).
All native checks, all five VM lanes, cloud-template checks and Sonar pass on e50f4a2. The known intermittent webhook plugin-fixture race (#893) recurred on the first attempt and passed on the rerun; the final Validate module & VM check is green. Evidence was added to that issue. No assertion was weakened. The seven CodeRabbit findings are addressed and their threads resolved; its review refresh was skipped due to the hourly allowance, so the earlier review verdict has not been refreshed.
The complete SMTP/STARTTLS VM integration passed on e50f4a2, including distinct failed MDA attempts before restoring the bus. Native first-boot domain discovery now updates implicit email domains; runtime changes restart intake; existing account/home mismatches fail before commit. Azure inbound priorities are checked for collisions. Permanent message refusals return 65, while service/handoff failures retain 75.
The SMTP VM found a Linux interface-discovery requirement: OpenSMTPD uses AF_NETLINK through getifaddrs. Both service backends now permit that family. The isolated smoke passes with netlink allowed and fails to start SMTP when it is denied. The VM startup wait now prints journals promptly on failure; the systemd/SMTP test has passed.
Add opt-in OpenSMTPD reception with STARTTLS and exact enabled-user recipients on NixOS and native hosts. Normalize MIME as the recipient uid and emit stable signed email events into the existing durable bus. Retained content and producer retry support depend on local-channels PR 107, pinned by immutable commit.
Add SMTP integration and adapter/render regressions, optional cloud ingress, and configuration/operational documentation. Preserve outbound SMTP restrictions and disable intake by default.
This change adds optional inbound SMTP for configured Linux users on native and NixOS hosts. Email is routed through each user’s existing webhook bus. It adds configuration and firewall controls, message limits, tests, and operator documentation.
Changes: Inbound email delivery
Layer / File(s)
Summary
Email configuration and validation bin/agentbox, modules/agent-box.nix, modules/agent-box.nix.in, tests/spec.nix, tests/native/config.*, tests/test_agentbox.py
Adds host and per-user email options, configuration validation, and generated native configuration fields.
defangdevs/agent-box#920 — This change updates the webhook pin and uses its durable inbox, emit, and read-content behavior.
Suggested reviewers:lionello
Merge Risk: Moderate · up to b9710
Optional inbound email has several configuration and lifecycle defects to fix before merge. Azure deployments that enable both email ingress and debug SSH will fail. Automatic-domain first boots can reject or mis-set the email domain. Updates may leave the old SMTP intake process running. Invalid domains or user names fail only at runtime. The default-off feature limits the blast radius.
Pre-merge checks: 3 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Linked Issues check
Issue #231 requirements are partly implemented. The adapter restricts envelope recipients and domains, uses the recipient uid, rejects relay and plus-tagged addresses, preserves MIME content, and retu…
Add and pass automated native and NixOS SMTP/STARTTLS integration coverage for the remaining #231 criteria. Cover enabled-user delivery, standing-watch and admission routing, refusal cases, receiver restart, absent-peer retry, duplicate han…
Passed checks: 3 passed
Check name
Status
Explanation
Title check
The title clearly and concisely describes the main change: optional SMTP intake for Linux users that delivers email to the message bus.
Out of Scope Changes check
The changes stay within issue #231. They add email configuration and delivery for NixOS and native hosts, optional cloud TCP 25 ingress, the MIME adapter, retained-content documentation, generated fix…
Description check
The pull request description directly explains the optional inbound email feature, its configuration, security boundaries, cloud ingress changes, limits, dependencies, and verification results. It mat…
Full details: Out of Scope Changes check
Explanation
The changes stay within issue #231. They add email configuration and delivery for NixOS and native hosts, optional cloud TCP 25 ingress, the MIME adapter, retained-content documentation, generated fixtures, and related regression and integration tests. The cloud rules are opt-in and support the requested inbound SMTP path. No unrelated feature or resource is demonstrated.
Full details: Description check
Explanation
The pull request description directly explains the optional inbound email feature, its configuration, security boundaries, cloud ingress changes, limits, dependencies, and verification results. It matches the changeset and objectives.
Full details: Linked Issues check
Explanation
Issue #231 requirements are partly implemented. The adapter restricts envelope recipients and domains, uses the recipient uid, rejects relay and plus-tagged addresses, preserves MIME content, and returns failure for bus handoff retry. The NixOS test covers SMTP rejection, bus downtime retry, privacy, retained content, and .forward bypass. However, the PR description states that real SMTP/STARTTLS VM integration is still pending. The reviewed test covers NixOS only and does not exercise the native backend, standing-watch admission, receiver restart recovery, duplicate handoff, full-disk handling, or capacity deferral. These are explicit #231 implementation acceptance criteria.
Resolution
Add and pass automated native and NixOS SMTP/STARTTLS integration coverage for the remaining #231 criteria. Cover enabled-user delivery, standing-watch and admission routing, refusal cases, receiver restart, absent-peer retry, duplicate handoff, full-disk and capacity deferral, user isolation, complete retained MIME content, and post-restart access.
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Use fixed root-managed executable symlinks and a fixed root-owned configuration file, removing executable paths and configuration paths from runtime input. Preserve GitHub as the default source when email is initialized at boot. Add configuration ownership and source initialization regression coverage.
The startup oneshot runs outside the agent supervisor, so explicitly provide the endpoint setup requires on both backends. Keep secret-bearing stdout discarded while retaining diagnostic stderr. Assert the native unit environment and expose startup failures in the SMTP VM test.
OpenSMTPD literal mailaddr tables also match subaddresses. Use anchored recipient expressions so plus-tagged recipients are rejected at SMTP time. Verified real SMTP/STARTTLS through the private bus and content store in an isolated rootless container, including two transactions sharing a Message-ID.
OpenSMTPD requires smtpctl to run with the smtpq effective group, including for root. Expose a fixed non-setgid tool link and use an explicit root/group switch in the VM retry test and operator documentation. No privileges are granted to agent users.
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Actionable comments posted: 7
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @bin/agentbox:
- Line 1008: Update the default email-domain assignment in the initialization
flow so it uses the domain resolved by first_boot() when email.domains is not
explicitly configured. Ensure auto or unset domains resolve to the derived
hostname, and that a configured suffix does not remain as the recipient domain
when first boot derives a different box hostname.
- Line 1430: Update the logic that sets t.email_restart to account for changes
to the agent-box-email or smtpd runtime as well as changes to email.json, so
activation restarts an already running intake service when either runtime is
replaced.
- Around line 1435-1437: Update the home-directory handling used by the email
source unit and delivery so both resolve webhook state from the same validated
home; either use one shared home value or make setup fail when the configured
home differs from an existing account’s passwd home. Locate the relevant setup
flow through ensure_user() and the unit renderer that uses u.home.
Review comments at @deploy/azure/agent-box.bicep:
- Line 276: Update the inbound priority for the email-ingress rule in the Bicep
source to an unused value distinct from the ssh rule, such as 1002, so both
rules can be enabled together; regenerate the corresponding JSON deployment
template to match.
Review comments at @modules/agent-box.nix.in:
- Around line 4805-4806: Update the `cfg.email.domains` assertion to reject
domains ending in a dot or hyphen, and update the `emailUsers` assertion to
validate each user name against the runtime username rules as well as requiring
`u.web.passwordHashFile`. Keep evaluation-time validation aligned with
`validate_config` in `email-cli.py` and make the assertion message describe both
user requirements.
Review comments at @modules/src/email-cli.py:
- Around line 189-191: Add a distinct exception for permanent message refusals
and raise it for the MIME-part, recipient, and message-size checks. In the
__main__ exception handling, catch that exception before the existing ValueError
clause and exit with status 65; preserve status 75 for configuration, identity,
and handoff failures.
Review comments at @tests/email.nix:
- Line 69: Update the email delivery test around the spool-content assertion to
wait until OpenSMTPD has recorded failed MDA delivery for both messages before
restoring the webhook socket and scheduling the retry; keep the subsequent retry
assertions dependent on those confirmed failures.
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Closes #231. Companion defangdevs/local-channels#107 has merged with green post-merge CI. Its reviewed immutable commit is pinned here.
When enabled, mail to
<linux-user>@<configured-hostname>reaches that user's existing message bus. OpenSMTPD handles SMTP/STARTTLS and durable transport retries; a shared MIME adapter runs as the recipient uid and publishesemail.received. Normal subscriptions, standing watches, profiles, ownership and admission handle the agent routing.Intake is disabled by default on both NixOS and native hosts. Only explicitly enabled Linux users and exact configured domains are accepted. An explicit userbase bypasses
.forward; envelope data never becomes an MDA command. The adapter preserves MIME and decoded alternatives/attachments in the companion's private retained payload store, using stable IDs and byte-identical JSON on retry. Mail headers and claimed senders remain untrusted; this does not authenticate email authors or add outbound replies.OpenSMTPD was chosen for its compact declarative fit to this reception-to-local-command path. Postfix could implement the same boundary; no performance/security superiority is claimed. Operators supply root-managed certificate/key files and restart intake after renewal. Existing web/webhook capability is required. Configuration and operational details are in
docs/email.md.Networking, migration and bounds
EC2/Lightsail
EmailIngressand AzureemailIngressoptionally open inbound TCP 25 on existing cloud firewalls (default false), independently of browser client CIDRs. Outbound SMTP restrictions stay in place. No new cloud resources or IAM grants are added. Enabling assumes port 25 is available; it does not replace an existing MTA or export Caddy's private key.Incoming MIME defaults to 2 MiB, with 100 MIME parts and per-connection transaction/recipient limits. Retained content is bounded separately (24 hours, 128 MiB, 2,000 records by default); general bus/MTA spool byte quotas remain the separate local-channels#105 work. Disabling stops intake and preserves transport queue files; accepted bus work follows normal routing/expiry.
Verification
.forwardoverride, private retained content and untrusted sender semantics..forwardbypass, bus downtime with MTA retry, per-user privacy and complete retained content after restart (passed in GitHub CI on e50f4a2; this host has no KVM).All native checks, all five VM lanes, cloud-template checks and Sonar pass on e50f4a2. The known intermittent webhook plugin-fixture race (#893) recurred on the first attempt and passed on the rerun; the final Validate module & VM check is green. Evidence was added to that issue. No assertion was weakened. The seven CodeRabbit findings are addressed and their threads resolved; its review refresh was skipped due to the hourly allowance, so the earlier review verdict has not been refreshed.
The complete SMTP/STARTTLS VM integration passed on e50f4a2, including distinct failed MDA attempts before restoring the bus. Native first-boot domain discovery now updates implicit email domains; runtime changes restart intake; existing account/home mismatches fail before commit. Azure inbound priorities are checked for collisions. Permanent message refusals return 65, while service/handoff failures retain 75.
The SMTP VM found a Linux interface-discovery requirement: OpenSMTPD uses AF_NETLINK through getifaddrs. Both service backends now permit that family. The isolated smoke passes with netlink allowed and fails to start SMTP when it is denied. The VM startup wait now prints journals promptly on failure; the systemd/SMTP test has passed.
No deployment or merge is part of this task.