“Unknown” status means the email could be valid or invalid but more so invalid than valid.
It just means that we used all our processes to try and validate that email address provided to us but we don't have sufficient evidence (50-50) to say if it’s valid or invalid.
In general, we highly recommend to stick with the ones we have states with certainty that they are valid to avoid unnecessary bounce rate. You can get rid of the unknown status easily by sorting it out through CSV (see our support center articles).
Our suggestion is to submit unknowns time to time so we can validate them again for you. Generally over time, things change and sometimes ESPs allow verification of those IDs that they didn't allow previously. This could reduce them further.