Also, an actual email client can specialise. BeFS is very general, the attributes can be typed but there's only one string type (other types include 32-bit and 64-bit integers, floats) so a Subject and a Sender email address both get indexed the same way. An email client might reasonably optimise for @corp.example type address searches for example, but then also do some simple keyword optimisations for subject line too.
You could not turn off indexing for some/ all folders in BeOS only for an entire filesystem (and if you do a bunch of stuff doesn't work)