The second one 'す' matched only 10 Japanese spam messages.
Another one which might match is Japanese punctuation, such as the comma 、 and the period 。
https://www.tofugu.com/japanese-grammar/particle-no-noun-mod...
I was not able to use comma 、 and the period 。 because I think FastMail disables searches on common punctuations, so those matched nothing.
(In case people are wondering, I sometimes scan through my Spam folder to check for false positives, i.e. things which were incorrectly marked as spam. It's difficult to do that when it is flooded with Japanese spam.)