So I had another thread asking about regex, but after some testing it seems to me that mail filters looking in headers do not work at all, period.
After trying a half dozen other things that didn't work, I put a test filter in to the effect "if headers contain 'mydomain' add TESTTEST to subject and stop".
This should match ALL my emails obviously, and put that prefix on every email I receive for a while, but it does absolutely nothing. Emails just come through completely untouched.
Are mail filters completely unfunctional? Is this completely broken?