spam.invisible_characters
The text contains zero-width, bidi-control or other invisible formatting characters.
What it looks at#
A curated set of Unicode characters that render as nothing or next to nothing: zero-width space/joiner/non-joiner, the zero-width no-break space (byte-order mark), word joiner, left-to-right/right-to-left marks, and the bidi embedding/override/isolate control characters. Not the full Unicode “format character” category — that also covers controls that are ordinary in real Arabic and other right-to-left text; this list is scoped to characters whose legitimate use in real messages is rare to nonexistent.
Ordinary whitespace is never flagged. Neither is a zero-width joiner used inside a real emoji sequence (the character that glues a family or profession emoji together, 👨👩👧👦) — only a zero-width joiner that isn’t sitting between two emoji characters counts.
Why it matters#
Invisible characters are a real evasion technique: splitting a blocked word so pattern matching misses it (“viagra” with zero-width spaces inside), or using bidi override characters to make text display differently than its actual character order — used to disguise file extensions and link destinations.
What to do#
Medium severity — this is a deliberate technique, not something that happens by accident in normal writing.