Either a spam filter so strict it blocks legit emails
or a relaxed spam filter that lets spams through.
In my experience, people complain far, far more about false-positives than about false-negatives. Greymail (IE the messages that go to your Spam folder rather than being dropped) are helpful with this, since false-positive senders can be added to your address book and implicitly whitelisted. However filters always err on the side of caution as (in general) people are happier that way.
Maybe if there is enough public demand for it, Spark will make the spam threshold user-configurable?
🔎 If you're up for it, you can possibly tighten your thresholds by writing your own Xtramail filters under Settings/Email/FilterRules by searching for the header SMX-S1S and dropping everything not 0, but be careful; if it were a magic solution then Spark would probably already be doing it