My suspicion is that the spam filter programmers didn't do a comprehensive evaluation of every code point on every plane of Unicode because... well that's a massive job. So your "sanitize and denormalize" tasks are actually massive mappings which were likely imperfectly created.
My suspicion is that the spam filter programmers didn't do a comprehensive evaluation of every code point on every plane of Unicode because... well that's a massive job. So your "sanitize and denormalize" tasks are actually massive mappings which were likely imperfectly created.