Remember that NONE of the results in any state are certified yet. Everything at this point is just projected wins based on statistical probability. Until all votes are counted, and potentially recounted depending on each state's voting laws, there are no official winners yet anywhere. Some states have mandatory recounts if results are within a certain range. Some the candidates can request recounts if within certain ranges. That sort of thing.

Most states allow mail in votes as long as they are postmarked by election day. Usually the race isn't close enough to where the likelihood of ballots arriving more than a day or so after would change the outcome. Mississippi is unusually close, it is extremely rare for any election to be this close.

It's not a big deal really, the entire electoral system timeframe in the US is still based on needing to travel by horse to deliver notifications of counts and things like that. It was never updated for modern technology so there is plenty of time built into the system to count and adjudicate normally.