Mail flow rules: carry out the transport actions

Phase 2g of the DLP and mail flow rules spec: transport rules now act,
on outgoing and incoming mail.

- features/mailflow/rewrite.rs: add or remove a header, prefix or set the
  subject (an RFC 2047 word when not ASCII), add a disclaimer. A
  disclaimer edits the message's main text and HTML bodies only, each
  decoded, changed and written back as UTF-8 quoted-printable with its
  other headers kept, top or bottom (after <body> or before </body> in
  HTML); attachments and attached messages are left alone, and a
  disclaimer already present isn't added again.
- smtp/inbound/mailflow.rs: the check runs for incoming mail too
  (transport rules only; DLP stays outgoing). After DLP passes, each
  matched transport rule's actions run in order: message edits,
  add-recipient and redirect (envelope changes DATA applies), route (a
  per-message queue ahead of the queue strategy), refuse (550 5.7.1
  with the rule's text). The override tag is stripped with the same
  subject writer, so a non-ASCII subject stays valid.
- Audit: refusals and changes to where mail goes are recorded (sender,
  or system:mail-flow for incoming mail); wording and header changes
  aren't, or a banner rule would record every message (spec §2.7).

Tests: rewrite unit tests (headers, encoded subjects, disclaimers on a
single part and on multipart/alternative with an attachment, once
only); mail_rules_tests gains the actions end to end: disclaimer,
header and subject prefix on a delivered message, a redirect, a
refusal, a banner on incoming LMTP mail that outgoing rules leave
alone, and which of those are audited.

crates/features/src/mailflow/rewrite.rs

@@ -0,0 +1,306 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Transport actions that change a message (§2.4): headers, the subject,
+//! disclaimers. Each takes the raw message and returns the new one, or
+//! `None` when there's nothing to change.
+//!
+//! Only what the action names changes. A disclaimer edits the message's
+//! main text and HTML bodies (not attachments, not attached messages):
+//! each is decoded, changed and written back as UTF-8 quoted-printable,
+//! with its other headers kept. A disclaimer already there isn't added
+//! again, so a reply thread carries it once.
+
+use base64::{Engine, engine::general_purpose::STANDARD};
+use mail_builder::encoders::quoted_printable::QuotedPrintableEncoder;
+use mail_parser::{HeaderName, MessageParser, PartType};
+
+use super::rules::Position;
+
+/// A header value, as an RFC 2047 encoded word when it isn't plain ASCII.
+pub fn header_value(value: &str) -> String {
+    if value.is_ascii() {
+        value.to_string()
+    } else {
+        format!("=?utf-8?B?{}?=", STANDARD.encode(value))
+    }
+}
+
+/// `Name: value` added at the top of the message.
+pub fn add_header(message: &[u8], name: &str, value: &str) -> Vec<u8> {
+    let mut out = Vec::with_capacity(message.len() + name.len() + value.len() + 4);
+    out.extend_from_slice(name.as_bytes());
+    out.extend_from_slice(b": ");
+    out.extend_from_slice(header_value(value).as_bytes());
+    out.extend_from_slice(b"\r\n");
+    out.extend_from_slice(message);
+    out
+}
+
+/// Every top-level header called `name` taken out.
+pub fn remove_header(message: &[u8], name: &str) -> Option<Vec<u8>> {
+    let parsed = MessageParser::new().parse_headers(message)?;
+    let mut ranges: Vec<(usize, usize)> = parsed
+        .headers()
+        .iter()
+        .filter(|h| h.name.as_str().eq_ignore_ascii_case(name))
+        .map(|h| (h.offset_field as usize, h.offset_end as usize))
+        .collect();
+    if ranges.is_empty() {
+        return None;
+    }
+    ranges.sort_unstable();
+    let mut out = Vec::with_capacity(message.len());
+    let mut at = 0;
+    for (start, end) in ranges {
+        out.extend_from_slice(&message[at..start]);
+        at = end;
+    }
+    out.extend_from_slice(&message[at..]);
+    Some(out)
+}
+
+/// The Subject header replaced by `subject` (added if there was none).
+pub fn set_subject(message: &[u8], subject: &str) -> Vec<u8> {
+    let line = format!("Subject: {}\r\n", header_value(subject));
+    let parsed = MessageParser::new().parse_headers(message);
+    match parsed
+        .as_ref()
+        .and_then(|p| p.headers().iter().find(|h| h.name == HeaderName::Subject))
+    {
+        Some(header) => {
+            let mut out = Vec::with_capacity(message.len() + line.len());
+            out.extend_from_slice(&message[..header.offset_field as usize]);
+            out.extend_from_slice(line.as_bytes());
+            out.extend_from_slice(&message[header.offset_end as usize..]);
+            out
+        }
+        None => {
+            let mut out = line.into_bytes();
+            out.extend_from_slice(message);
+            out
+        }
+    }
+}
+
+/// `prefix` put before the subject, unless it's already there.
+pub fn prefix_subject(message: &[u8], prefix: &str) -> Option<Vec<u8>> {
+    let parsed = MessageParser::new().parse_headers(message)?;
+    let subject = parsed.subject().unwrap_or_default();
+    if subject.trim_start().starts_with(prefix.trim()) {
+        return None;
+    }
+    Some(set_subject(
+        message,
+        &format!("{} {}", prefix.trim(), subject.trim_start()),
+    ))
+}
+
+fn escape_html(text: &str) -> String {
+    text.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('\n', "<br>\n")
+}
+
+fn with_text_disclaimer(body: &str, text: &str, position: Position) -> String {
+    let text = text.trim_end();
+    match position {
+        Position::Top => format!("{text}\r\n\r\n{body}"),
+        Position::Bottom => format!("{}\r\n\r\n{text}\r\n", body.trim_end()),
+    }
+}
+
+fn with_html_disclaimer(body: &str, html: &str, position: Position) -> String {
+    let lower = body.to_ascii_lowercase();
+    match position {
+        Position::Top => match lower
+            .find("<body")
+            .and_then(|at| lower[at..].find('>').map(|end| at + end + 1))
+        {
+            Some(at) => format!("{}{html}{}", &body[..at], &body[at..]),
+            None => format!("{html}{body}"),
+        },
+        Position::Bottom => match lower.rfind("</body>") {
+            Some(at) => format!("{}{html}{}", &body[..at], &body[at..]),
+            None => format!("{body}{html}"),
+        },
+    }
+}
+
+/// The disclaimer added to each main text and HTML body. `html` is the HTML
+/// version, or the text escaped when there's none.
+pub fn add_disclaimer(
+    message: &[u8],
+    text: &str,
+    html: Option<&str>,
+    position: Position,
+) -> Option<Vec<u8>> {
+    let parsed = MessageParser::new().parse(message)?;
+    let html = html
+        .map(str::to_string)
+        .unwrap_or_else(|| format!("<p>{}</p>", escape_html(text.trim())));
+    let marker = text.trim();
+
+    let mut body_parts: Vec<u32> = parsed
+        .text_body
+        .iter()
+        .chain(parsed.html_body.iter())
+        .copied()
+        .collect();
+    body_parts.sort_unstable();
+    body_parts.dedup();
+
+    // (start, end, replacement) for each part, applied from the last
+    let mut edits: Vec<(usize, usize, Vec<u8>)> = Vec::new();
+    for id in body_parts {
+        let Some(part) = parsed.parts.get(id as usize) else {
+            continue;
+        };
+        let (new_body, content_type) = match &part.body {
+            PartType::Text(body) => {
+                if body.contains(marker) {
+                    continue;
+                }
+                (with_text_disclaimer(body, text, position), "text/plain")
+            }
+            PartType::Html(body) => {
+                if body.contains(marker) || body.contains(html.as_str()) {
+                    continue;
+                }
+                (with_html_disclaimer(body, &html, position), "text/html")
+            }
+            _ => continue,
+        };
+        // The part's own headers, less the two this changes
+        let mut headers = Vec::new();
+        for header in part.headers() {
+            if matches!(
+                header.name,
+                HeaderName::ContentType | HeaderName::ContentTransferEncoding
+            ) {
+                continue;
+            }
+            headers.extend_from_slice(
+                &message[header.offset_field as usize..header.offset_end as usize],
+            );
+        }
+        headers.extend_from_slice(
+            format!("Content-Type: {content_type}; charset=utf-8\r\n").as_bytes(),
+        );
+        headers.extend_from_slice(b"Content-Transfer-Encoding: quoted-printable\r\n\r\n");
+        let encoded = QuotedPrintableEncoder::new()
+            .preserve_line_breaks()
+            .encode(new_body.as_bytes())
+            .ok()?;
+        headers.extend_from_slice(&encoded);
+        // A single-part message's headers are the message's: its first
+        // header is where the part starts
+        let start = part.headers().first().map_or(part.offset_header, |h| {
+            h.offset_field.min(part.offset_header)
+        }) as usize;
+        edits.push((start, part.offset_end as usize, headers));
+    }
+    if edits.is_empty() {
+        return None;
+    }
+    edits.sort_by_key(|(start, _, _)| std::cmp::Reverse(*start));
+    let mut out = message.to_vec();
+    for (start, end, replacement) in edits {
+        out.splice(start..end.min(out.len()), replacement);
+    }
+    Some(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn parse(message: &[u8]) -> mail_parser::Message<'_> {
+        MessageParser::new().parse(message).expect("parses")
+    }
+
+    const PLAIN: &[u8] = b"From: [email protected]\r\nTo: [email protected]\r\nSubject: Hello\r\nContent-Type: text/plain; charset=iso-8859-1\r\nContent-Transfer-Encoding: quoted-printable\r\n\r\nCaf=E9 at noon.\r\n";
+
+    const ALTERNATIVE: &[u8] = b"From: [email protected]\r\nSubject: Plans\r\nMIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=\"outer\"\r\n\r\n--outer\r\nContent-Type: multipart/alternative; boundary=\"inner\"\r\n\r\n--inner\r\nContent-Type: text/plain\r\n\r\nSee you.\r\n--inner\r\nContent-Type: text/html\r\nContent-Transfer-Encoding: base64\r\n\r\nPGh0bWw+PGJvZHk+PHA+U2VlIHlvdS48L3A+PC9ib2R5PjwvaHRtbD4=\r\n--inner--\r\n--outer\r\nContent-Type: text/plain; name=\"notes.txt\"\r\nContent-Disposition: attachment; filename=\"notes.txt\"\r\n\r\nAttachment text.\r\n--outer--\r\n";
+
+    #[test]
+    fn headers() {
+        let added = add_header(PLAIN, "X-Mail-Rule", "External");
+        assert_eq!(
+            parse(&added).header_raw("X-Mail-Rule").map(str::trim),
+            Some("External")
+        );
+        let removed = remove_header(&added, "x-mail-rule").unwrap();
+        assert_eq!(removed, PLAIN);
+        assert!(remove_header(PLAIN, "X-Absent").is_none());
+        let utf8 = add_header(PLAIN, "X-Note", "Überprüft");
+        // An RFC 2047 word: mail readers decode it, the wire stays ASCII
+        assert_eq!(
+            parse(&utf8).header_raw("X-Note").map(str::trim),
+            Some("=?utf-8?B?w5xiZXJwcsO8ZnQ=?=")
+        );
+    }
+
+    #[test]
+    fn subjects() {
+        let prefixed = prefix_subject(PLAIN, "[External]").unwrap();
+        assert_eq!(parse(&prefixed).subject(), Some("[External] Hello"));
+        assert!(prefix_subject(&prefixed, "[External]").is_none());
+        let accented = set_subject(PLAIN, "Réunion à midi");
+        assert_eq!(parse(&accented).subject(), Some("Réunion à midi"));
+        assert!(accented.is_ascii(), "encoded as an RFC 2047 word");
+        let none = set_subject(b"From: [email protected]\r\n\r\nBody\r\n", "New");
+        assert_eq!(parse(&none).subject(), Some("New"));
+    }
+
+    #[test]
+    fn disclaimer_on_a_single_part() {
+        let out = add_disclaimer(PLAIN, "Sent by Example Co.", None, Position::Bottom).unwrap();
+        let parsed = parse(&out);
+        let body = parsed.body_text(0).unwrap();
+        assert!(body.starts_with("Café at noon."), "{body:?}");
+        assert!(body.trim_end().ends_with("Sent by Example Co."), "{body:?}");
+        assert_eq!(parsed.subject(), Some("Hello"));
+        assert_eq!(
+            parsed.header_raw("To").map(str::trim),
+            Some("[email protected]")
+        );
+        // Once only
+        assert!(add_disclaimer(&out, "Sent by Example Co.", None, Position::Bottom).is_none());
+    }
+
+    #[test]
+    fn disclaimer_on_alternatives_leaves_attachments() {
+        let out = add_disclaimer(
+            ALTERNATIVE,
+            "Confidential.",
+            Some("<p><i>Confidential.</i></p>"),
+            Position::Top,
+        )
+        .unwrap();
+        let parsed = parse(&out);
+        assert!(
+            parsed
+                .body_text(0)
+                .unwrap()
+                .starts_with("Confidential.\r\n\r\nSee you."),
+            "{:?}",
+            parsed.body_text(0)
+        );
+        let html = parsed.body_html(0).unwrap();
+        assert!(
+            html.contains("<body><p><i>Confidential.</i></p><p>See you.</p>"),
+            "{html}"
+        );
+        assert_eq!(parsed.attachment_count(), 1);
+        assert_eq!(
+            parsed.attachment(0).unwrap().text_contents(),
+            Some("Attachment text.")
+        );
+        assert!(!String::from_utf8_lossy(&out).contains("Confidential.\r\n\r\nAttachment"));
+    }
+}