Explain this: the local model reads delivery failures, verdicts, logs and settings

A new method, inbuxa:Explanation/set, asks the node's local model for a
short plain-words reading of one thing an administrator is looking at:
a failed recipient in the queue, a Classify verdict, a log line or trace
event, or one setting with its saved value. The server builds the prompt
itself from stored data and the registry schema, never from text the
console sends, and grounds SMTP replies in RFC 3463 and RFC 5321.

What the model is never shown: secrets (including ones nested inside a
setting, like an AI model's HTTP auth), raw protocol events, and the
contents of any other event. A tag name that doesn't have a tag's shape is
refused before a model is asked.

Calls share the AI gate with spam classification, but mail always keeps
its slot, and Explain has its own hourly count per account and its own
on/off switch in inbuxa:AiLimits. The permission is sysAiExplain,
superuser only; tenant administrators can't use it. The session carries
an aiExplain flag so a console knows when to offer the button.

An install whose roles were stored before the permission existed gets it
added once, at start-up, to the roles that are administrators' alone,
not the User role their defaults share with every account. An operator
who removes it later isn't overruled.

Tests: unit tests in inbuxa-features and jmap, and ai_explain_tests
(run with --ignored) covering the acceptance tests and the upgrade.

crates/features/src/ai/explain/status.rs

@@ -0,0 +1,115 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Reference notes on SMTP replies for explaining a delivery failure (EX-7),
+//! in this project's own words, from RFC 5321 §4.2 (reply codes), RFC 3463
+//! (enhanced status codes) and the codes later RFCs registered (RFC 7372,
+//! RFC 7505).
+
+/// Notes for a basic reply code and an enhanced code, as far as they are
+/// known. Unknown parts add nothing.
+pub fn notes(code: Option<u16>, enhanced: Option<&str>) -> Vec<String> {
+    let mut notes = Vec::new();
+    let class = enhanced
+        .and_then(|e| e.split('.').next())
+        .and_then(|c| c.parse::<u8>().ok())
+        .or_else(|| code.map(|c| (c / 100) as u8));
+    match class {
+        Some(2) => notes.push("A 2xx reply or class 2 status means success.".to_string()),
+        Some(4) => notes.push(
+            "A 4xx reply or class 4 status is a temporary failure: the sending server keeps \
+retrying until its retry period ends, and the same message may later go through."
+                .to_string(),
+        ),
+        Some(5) => notes.push(
+            "A 5xx reply or class 5 status is a permanent failure: retrying the same message \
+won't help until something changes, and the sender is sent a bounce."
+                .to_string(),
+        ),
+        _ => {}
+    }
+    let Some(enhanced) = enhanced else {
+        return notes;
+    };
+    let mut parts = enhanced.split('.');
+    let (_, subject, detail) = (parts.next(), parts.next(), parts.next());
+    if let Some(note) = subject.and_then(|s| s.parse::<u16>().ok()).and_then(subject_note) {
+        notes.push(note.to_string());
+    }
+    if let (Some(subject), Some(detail)) = (subject, detail)
+        && let Some(note) = detail_note(subject, detail)
+    {
+        notes.push(format!("x.{subject}.{detail}: {note}"));
+    }
+    notes
+}
+
+fn subject_note(subject: u16) -> Option<&'static str> {
+    Some(match subject {
+        0 => "Subject x.0 is 'other or undefined': the code alone says little; the reply text matters.",
+        1 => "Subject x.1 concerns the address: the mailbox or domain named in the envelope.",
+        2 => "Subject x.2 concerns the recipient's mailbox itself: full, disabled, or refusing.",
+        3 => "Subject x.3 concerns the receiving mail system: its capacity, configuration or features.",
+        4 => "Subject x.4 concerns the network or routing: DNS, connections, or loops.",
+        5 => "Subject x.5 concerns the SMTP conversation: a command or its order was refused.",
+        6 => "Subject x.6 concerns the message's content or format.",
+        7 => "Subject x.7 concerns security or policy: authentication checks, reputation, or rules on the receiving side.",
+        _ => return None,
+    })
+}
+
+fn detail_note(subject: &str, detail: &str) -> Option<&'static str> {
+    Some(match (subject, detail) {
+        ("1", "1") => "the mailbox doesn't exist at the receiving domain",
+        ("1", "2") => "the recipient's domain doesn't exist or can't receive mail",
+        ("1", "3") => "the recipient address isn't valid",
+        ("1", "10") => "the domain publishes a null MX: it accepts no mail",
+        ("2", "1") => "the mailbox is disabled or not accepting mail",
+        ("2", "2") => "the mailbox is full",
+        ("2", "3") => "the message is larger than this mailbox accepts",
+        ("3", "4") => "the message is larger than the receiving system accepts",
+        ("4", "1") => "no answer from the receiving host",
+        ("4", "2") => "the connection was lost or refused",
+        ("4", "3") => "a directory or DNS lookup failed",
+        ("4", "4") => "no route to the destination: often a missing or broken MX record",
+        ("4", "6") => "a mail loop was detected",
+        ("4", "7") => "delivery took too long and expired",
+        ("5", "3") => "too many recipients for one message",
+        ("7", "0") => "refused for a security or policy reason not given more precisely",
+        ("7", "1") => "the receiving server's policy doesn't allow this delivery",
+        ("7", "8") => "authentication credentials were refused",
+        ("7", "23") => "the sender's SPF check failed",
+        ("7", "24") => "the SPF check couldn't be completed",
+        ("7", "25") => "the sending IP's reverse DNS check failed",
+        ("7", "26") => "several authentication checks failed together, typically SPF and DKIM, so DMARC failed",
+        ("7", "27") => "the sender's domain publishes a null MX, so it can't receive the bounce",
+        ("7", "28") => "the sender is sending too much mail to this receiver",
+        _ => return None,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn notes_for_a_dmarc_rejection() {
+        let n = notes(Some(550), Some("5.7.26"));
+        assert_eq!(n.len(), 3);
+        assert!(n[0].contains("permanent"));
+        assert!(n[1].starts_with("Subject x.7"));
+        assert!(n[2].starts_with("x.7.26:"));
+    }
+
+    #[test]
+    fn partial_and_unknown() {
+        assert_eq!(notes(Some(421), None).len(), 1);
+        assert!(notes(None, None).is_empty());
+        let n = notes(None, Some("4.9.99"));
+        assert_eq!(n.len(), 1);
+        assert!(n[0].contains("temporary"));
+    }
+}