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:

Inbound message

URLs extracted

Reputation cache

confirmed good / bad hosts

MISUIC

RULE 0-5 ladder

Worst verdict

across all URLs

Mail proxy

scores and routes

A verdict of "dangerous" or "safe" is scored deliberately low: MISUIC's own symbols sit below the threshold that alone would add a warning header, so it corroborates the wider pipeline's judgement rather than overriding it outright.

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.

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.com must never hand out trust to every other subdomain of example.com.

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. A "safe" verdict is only reachable when every URL in the message is safe; a single dangerous URL is enough to outrank the rest.