How MISUIC reaches a verdict
Every rule below is deterministic: given the same URL and the same cached reputation, MISUIC returns the same answer every time, and nothing on this ladder calls a model. That makes a rule ladder verdict a fast, repeatable first-pass signal, not a final word: it is not conclusive on its own. A URL that stays inconclusive after every rule below rests at "unknown." MISUIC is built the way Michelle builds this whole ecosystem: run cheaply on modest, older hardware first, then add newer hardware and GPU-backed checks deliberately, for accuracy and coverage, where they earn their keep rather than by default. A GPU-backed cross-check for exactly the URLs that stay inconclusive here is the next such step, planned rather than live today.
Where it sits
MISUIC runs as a small, standalone scanner registered with the mail proxy's own filtering engine, checked against every URL a message contains before the message is scored:
"Dangerous" is the one verdict that decides on its own: it scores above the threshold that alone adds a warning, and it is not overridable downstream. "Malicious," "spam" and "safe" all sit below that threshold, so they corroborate the wider pipeline's judgement rather than overriding it outright, and malicious/spam are the two severities that stay overridable further down the pipeline.
The rule ladder
A URL is checked against each rule in order. Rules 2 through 4 look for structural impersonation; rule 5 only ever corroborates one of them, and never condemns a URL by itself; rule 0 is the only path to a "safe" verdict, and only when nothing else has already condemned the URL. Every rule that can condemn a URL below lands on the top severity, "dangerous," regardless of which one fired: impersonating a brand is scam/phishing behaviour by nature, so structure alone earns the top tier. The graduated "malicious" and "spam" severities come from a different source entirely -- see the next section.
| Rule | Checks | Can it condemn alone? |
|---|---|---|
| RULE 0 | Safe must be earned. The URL's registrable domain is checked against known brand domains and the confirmed-good side of the reputation cache. No positive evidence means "unknown," not "safe." | Withholds, never condemns |
| RULE 1 | A claimed brand, supplied by the caller, is checked against that brand's own known domains. | Yes |
| RULE 2 | Purely deterministic, no model involved: a brand's own name or alias appears inside a domain that does not belong to that brand. | Yes |
| RULE 3 | Confusable and IDN homograph matching: catches a lookalike domain built from characters that render the same but are not the same, such as a Cyrillic letter standing in for a Latin one. | Yes |
| RULE 4 | Bounded edit-distance matching against a brand's registered domain: catches a close typo of a real domain. | Corroborates only |
| RULE 5 | Domain age as corroboration: a very young domain strengthens an otherwise inconclusive rule 1, 2 or 3 finding. | Corroborates only |
Reading the reputation cache
MISUIC shares a single host-keyed reputation cache with the wider content-classification pipeline it sits in front of, so a host confirmed once by either side is never re-litigated by the other. Two rules apply strictly:
- Only confirmed entries count. An unconfirmed sighting, one that has not yet been corroborated, never changes a verdict on its own.
- A confirmed-good host is matched as that exact host, never widened to
its registrable parent domain. Trusting
mail.example.commust never hand out trust to every other subdomain ofexample.com.
Grading a confirmed-bad host
A confirmed-bad host carries its own recorded category, and that category is what decides its severity, rather than every confirmed-bad host flooring at "dangerous" the way it used to:
| Recorded category | Severity |
|---|---|
| Phishing, malware | Dangerous |
| Advance-fee scam, extortion | Malicious |
| Bulk marketing, click spam | Spam |
| No category recorded (older manual entries, mostly) | Dangerous |
A host with no category on file floors at "dangerous" rather than quietly downgrading it: that is the conservative default, and it is the same strictness every confirmed-bad host had before this grading existed.
The fail-open contract
Any internal error, a cache that cannot be reached, a malformed request, an unexpected exception, is MISUIC's own problem and is reported as such. The mail path continues exactly as if MISUIC were not there: it can only ever add caution, never block delivery by failing.
Worst wins
A message can contain many URLs. MISUIC checks every one and reports the single worst rating found, with the URL and the rule that produced it. The severity ladder, from best to worst, is safe/none, unknown, spam, malicious, dangerous: a "safe" verdict is only reachable when every URL in the message is safe, and a single dangerous URL outranks every other URL in the message, whatever their own ratings.