inbuxa-server/docs/spec/features/dlp-and-mail-flow-rules.md

Feature spec: data loss prevention and mail flow rules

Status: draft for approval. Phase 1 of DLP and the rule builder, specced together because they need the same conditions, the same place in the mail path and the same record of what matched. Not a rebuild of an upstream feature, so it has no line in SPEC.md §4's table. Open questions are under For John; nothing is built until they're answered.

Provenance

Written for the record SPEC.md §3 rule 3 asks for. Sources, and nothing else:

Source	License	Used for
This repository at 0502eb4 (2026-09-28): crates/smtp/src/inbound/data.rs, crates/smtp/src/queue/, crates/jmap/src/submission/set.rs, crates/common/src/scripts/, vendor/sieve-rs, resources/schema/schema.json.gz	AGPL-3.0-only	Where a check can run, what the queue stores, what a sender sees on a refusal
inbuxa-admin at b82904c	AGPL-3.0-only	Where the pages go
ihasmail-inbuxa (the webmail) at 290bc63	AGPL-3.0-or-later	How a refused send reaches the person sending
inbuxa-drafts/queue/dlp.md, rule-builder.md	Own	What John asked for and settled
The personal-data catalog spec and the audit-hold-lock spec	Own	Roles, the audit log, legal holds, the catalog check
RFC 5321, RFC 3463 (enhanced status codes), RFC 8620/8621 (JMAP)	Public	Refusal codes and the submission error shape

No Enterprise-only file or snippet was used, and no third-party DLP product was consulted for design: the detectors are public checksum and format rules (Luhn, ISO 13616 mod 97, the SSA's published SSN rules).

What it is

1. Data loss prevention (DLP). Policies that look at mail as someone sends it, find what shouldn't leave (card numbers, bank accounts, national ID numbers, words and patterns an organization names, attachments of a kind), and then block it with a notice, warn and let the sender send anyway with a stated reason, or hold it until a reviewer releases or rejects it.
2. Mail flow rules. The same engine, for ordinary transport rules an administrator writes in a form instead of in Sieve: disclaimers, banners, headers, copies, redirects, refusals.
3. One record of what matched, in the audit log, and a review queue for held mail.

Settled before this spec (John, 2026-09-27 and 2026-09-28): DLP's first version checks outgoing mail only, content and attachments, as it's sent; its actions are block with a notice, warn with an override (audited), and hold for review. A record-only action was not chosen. The rule builder goes alongside DLP, one design; journaling comes after both.

Out of scope (later specs): inbound DLP, files and calendar sharing, scanning mail already stored, a machine-learning classifier (the local AI model could add one later; nothing here depends on it), mobile and ihasmail screens, journaling.

Nothing in code, docs, UI text or output claims the product meets a legal standard or prevents every leak. The pages say what a policy checks and what it did.

1. What exists today

Checked by reading the code at 0502eb4:

Need	Today
A place in the send path that sees every outgoing message	Yes. SMTP submission and webmail sends both reach Session::queue_message (inbound/data.rs); JMAP submission builds a local session and runs MAIL, RCPT and DATA (jmap/src/submission/set.rs ~L690–760). Nothing reaches the queue around it.
Order at DATA	Authentication checks → spam filter → milters → MTA hooks → the DATA system Sieve script → headers, DKIM signing → queue.
Rules without code	System Sieve (x:SieveSystemScript): one script per stage, chosen by an expression on x:MtaStageData.script. Hand-written only; the console has a text field.
Refusing a message	A 5xx at DATA. The webmail gets forbiddenToSend with the text Server rejected DATA: <reply> and nothing structured.
Holding a message	Nowhere. "Quarantine" in the code is only DMARC's disposition. The queue's recipient status is Scheduled, Completed, TemporaryFailure, PermanentFailure, archived with rkyv.
Reading attachments	Text and HTML parts only. There's no PDF or Office text extraction: search indexes text parts and file names.
Recording what happened	The audit log, with system actors, reasons and outcomes (AU-1…AU-12).
Who is allowed	Roles with per-permission grants; server and tenant levels; the compliance roles from the catalog spec.

2. Design

2.1 One engine, native, after the system script

Rules are evaluated by a new native engine at DATA, after the system Sieve script and before headers and DKIM signing. Mail flow rules and DLP policies are two views of the same rule list.

Why not generate Sieve: holding a message, a warning the sender can override, counted detectors with checksums, and a per-rule match record are all things Sieve doesn't have. Adding them means new extensions in vendor/sieve-rs, which widens the fork of a crate we'd otherwise take from upstream, and a generated script would have to share the single DATA script with whatever an administrator wrote by hand. A native engine leaves hand-written Sieve exactly as it is: it still runs, first, and the rules see its result.

The engine lives in crates/features/src/mailflow/ (pure evaluation over a parsed message and an envelope, unit-testable), called from data.rs behind one // inbuxa: marked block.

2.2 Rules

A fork-owned JMAP object, inbuxa:MailRule, stored in inbuxa's own subspace like legal holds (not a registry object, so upstream schema imports never touch it):

Property
name, description
kind	dlp or transport: which page shows it and which permission edits it
enabled
priority	Order; lower runs first
direction	outgoing (authenticated senders), incoming, or any. DLP rules are outgoing only in this version.
conditions	All must match (list below)
exceptions	Any matching one skips the rule
actions	What happens (list below)
stopProcessing	Later rules don't run for this message
tenantId	Server-level (none) or one tenant's; see For John Q3
createdBy, updatedAt

Every create, update and delete is audited with its before and after, like any setting, and appears on the compliance Overview's Changes that affect review.

2.3 Conditions

Shared by both kinds:

Condition	Matches when
Sender	the sender is one of the chosen accounts, or in a chosen group, domain or tenant
Recipient	any recipient is one of the chosen addresses, domains, groups
Recipient outside	any recipient isn't at a domain this server hosts (or, with a tenant rule, isn't in the tenant)
Subject or body contains	any of a list of words or phrases (whole words, case-insensitive)
Subject or body matches	a regular expression (the regex crate: linear time, no backtracking)
Header	a header exists, or its value contains or matches
Attachment	its detected type, extension or name matches; its size is over a limit; there are more than N
Can't be inspected	an attachment is encrypted or password-protected (ZIP, PDF, Office), or bigger than the inspection limit
Message size	over a limit

DLP adds detectors, each with a minimum count (for example "5 or more card numbers"):

Detector	Rule
Payment card number	13–19 digits, spaces or dashes allowed, a known issuer prefix, passes Luhn
IBAN	country code, check digits and length for that country, passes ISO 13616 mod 97
US Social Security number	AAA-GG-SSSS or nine digits beside words like "SSN"; never area 000, 666 or 9xx, group 00, serial 0000
Word list	a list the organization maintains (project names, "confidential"), counted
Pattern	the organization's own regular expression, counted

What the detectors read: the subject, every text and HTML part (as text), and attachments whose detected type is text (text/*, CSV, JSON, XML). Office and PDF files are read only if Q2 says so; until then they count as can't be inspected so a policy can still act on them. Inspection stops at a limit per message (proposed 10 MB of text), and what's past it counts as can't be inspected too.

2.4 Actions

Transport actions (both kinds): add a disclaimer (text and HTML, top or bottom, once per thread), add or remove a header, prefix the subject, add a recipient (a copy), redirect to other recipients, refuse with a text, send through a chosen route (an existing x:MtaVirtualQueue).

DLP actions, exactly one per DLP rule:

Action	Sender sees	Message
Block	SMTP 550 5.7.1 with the rule's notice text; in the webmail, the notice in the send dialog	Not accepted; nothing is stored
Warn	The notice and, once they give a reason, can send anyway	Sent after an override; the reason is audited
Hold for review	Accepted with "held for review"; a notice mail from the server if the rule asks	Waits in the queue for a reviewer

When several DLP rules match, the strictest wins: block, then hold, then warn.

2.5 Warn and override

Webmail (JMAP). The first submission fails with a new error type, inbuxa:dlpWarning, carrying the matched rules' names and notice texts (never the matched text). The webmail shows them and asks for a reason; it resubmits with inbuxa:dlpOverride: {"reason": "..."} on the EmailSubmission create (capability urn:inbuxa:jmap). The server passes the reason into the local SMTP session as trusted session data, not as a header, so it can't be forged from the message. An override covers only the rules that warned; if a block or hold rule also matches, that still applies.

Mail apps (SMTP). They show whatever text the server returns, so the refusal says how to override: 550 5.7.1 <notice>. To send anyway, start the subject with [override: your reason]. On the next attempt the engine strips the tag before DKIM signing, and records the reason. See Q1.

A block's refusal uses the same error path with inbuxa:dlpBlocked in the webmail.

2.6 Hold for review

A held message is queued normally but not scheduled: its due time is set to never, and a review record inbuxa:HeldMessage (queue id, sender, recipients, subject, size, matched rules and counts, held at, expires at) is written in inbuxa's own subspace. The queue's stored format is untouched, so a node still on the previous version during a rolling upgrade reads the message fine and simply never sends it.

The sender gets 250 2.0.0 Held for review, and the rule may send them a notice mail. The message stays in their Sent folder as usual.

A reviewer, under Management › Compliance › Held mail:

• sees the list, and opens one to read it (each opening is audited, like any access to someone else's mail);
• releases it with a reason: it's scheduled at once and delivered as normal;
• rejects it with a reason: it's removed from the queue and the sender gets a notice with the reviewer's note, not their name.

Unreviewed mail is rejected after a set time (Q5), with a notice to the sender. Emails › Queue shows held mail as held and refuses Retry on it, so nobody can deliver it around the review. The sender can't unsend it either once it's held (the webmail says so).

Held messages count against no one's quota. Each held message and each decision is in the audit log.

2.7 What's recorded

Every DLP match writes one audit record: actor DLP (a system actor), target the message (queue id, sender, recipient domains), the rules and each detector's count, the action, and for an override the sender's reason. Never the matched text: the log would otherwise become a second copy of what the policy was keeping in. A card number isn't written, even masked.

Transport rules that change a message record the rule and action the same way. Unmatched mail writes nothing.

2.8 Permissions and who does what

New permissions (ids from 674):

Permission	Gives
sysMailRuleGet / Update	See / change transport rules
sysDlpPolicyGet / Update	See / change DLP rules
sysDlpReviewGet	See held mail and open it
sysDlpReviewUpdate	Release or reject held mail

Proposed defaults: Administrator has all; Compliance Officer (server-level) has sysDlpPolicyGet, sysDlpReviewGet, sysDlpReviewUpdate; tenant roles per Q3. See Q4 on who edits DLP rules.

2.9 Privacy catalog

New entries, so the catalog check passes: inbuxa:MailRule (administrator identities), inbuxa:HeldMessage (sender, recipients, subject: held until reviewed or expired, then removed), and the held message's content in the queue (content, the sender's and correspondents'). The DLP audit records are covered by the audit log's entry.

2.10 Mixed versions and clusters

Rules and review records live in the shared data store, so every node sees the same ones. During a rolling upgrade a node still on the old version doesn't check mail against rules; the Overview can't tell. The console says so when nodes report different versions, and the release notes say to enable DLP rules after every node is upgraded.

2.11 Cost

Rules are compiled once when they change (regexes, word lists as an Aho-Corasick automaton) and shared by every session. Detectors only run on mail that some enabled rule could match (direction, sender, recipient checks first). The inspection limit caps the worst case.

3. Console

• Management › Compliance › Data loss prevention: DLP rules, a form with conditions, exceptions, detectors and the action; the notice text; what matched in the last 30 days (from the audit log).
• Management › Compliance › Held mail: the review queue.
• Settings › Mail flow › Rules (with the settings reorganization's approved order): transport rules, same form, ordered, with Stop processing.

Every form previews the rule in words ("If a recipient is outside and the message contains 5 or more card numbers, hold it for review").

4. Webmail (ihasmail-inbuxa)

• A warning dialog: the notice, a reason field, Send anyway and Edit message.
• A block dialog with the notice.
• A held message shows as Held for review in Sent, and its undo is gone.

5. Tests

Unit: each detector against valid and near-miss numbers (Luhn-failing cards, IBANs with a wrong check, SSN areas 000/666/9xx), word lists, regexes, the inspection limit, the can't-be-inspected cases, rule order and stop processing. Integration (tests/src/smtp/, tests/src/jmap/): block, warn and override over SMTP and JMAP, hold then release and reject, expiry, Retry refused on held mail, audit records carrying no matched text, a message queued by a node without the engine (held message format unchanged).

6. Phases

1. This spec, approved.
2. Engine, conditions, detectors, transport actions; DLP block and warn over SMTP and JMAP; audit records; catalog entries.
3. Hold for review: review records, release, reject, expiry, queue guard.
4. Console: DLP rules, held mail, mail flow rules.
5. Webmail dialogs; docs; a row in inbuxa-drafts/divergence-log.md.

Each phase is its own PR with tests; releases as John decides.

Known gaps

• Mail a user's own filter forwards automatically to an outside address isn't checked in this version (it leaves as generated mail, not a submission).
• What a mail app keeps in its own Sent folder, or sends through another server, is outside what this server sees.
• Detectors find formats, not meaning: a card number in a harmless test message matches; a number written in words doesn't.

For John

1. Override from mail apps. Recommended: the [override: reason] subject tag, stripped before sending. Alternatives: no override outside the webmail (mail apps get warn rules as blocks), or a header only a few apps can set.
2. Office and PDF attachments. Recommended: read DOCX, XLSX, PPTX and ODF in this version (ZIP plus XML, done in-house, no new service), PDF later. Alternative: all of them count as can't be inspected until a later spec.
3. Tenants. Recommended: server-level rules only in this version, with a tenant condition, so a service provider sets policy for everyone. Alternative: tenant administrators write rules for their own tenant too (more to build: a tenant ceiling on actions, a tenant's review queue).
4. Who edits DLP rules. Recommended: administrators edit, compliance officers see rules and review held mail, keeping "officers change no setting" true. Alternative: officers edit DLP rules too.
5. Unreviewed held mail. Recommended: rejected back to the sender after 7 days, with a notice. Alternatives: a different number, or held until someone decides.
6. Detectors in the first version. Recommended: the five above. Others (UK NI number, EU national IDs, passport numbers) are a table entry each later; name any that must be there from the start.