My understanding is that in most areas without existing voter suppression efforts anything postmarked by election day should be counted. This will probably delay the announcement of the winner unless it is a landslide, which is not really that big of a deal unless you are a cable new channel that feels it is necessary to have 24/7 coverage of the election until the winner is announced.
It may still be helpful to delay the counting of these votes to further the narrative "look, Trump won by a large margin the first day, then all these mail in votes come in a week later and the winner is now Biden? Obvious mail fraud!"
Yeah, other people are seeing that as his strategy, too:
https://www.nytimes.com/2020/08/11/opinion/trump-election-da...
> There’s no mystery about what President Trump intends to do if he holds a lead on election night in November. He’s practically broadcasting it.
> First, he’ll claim victory. Then, having spent most of the year denouncing vote-by-mail as corrupt, fraudulent and prone to abuse, he’ll demand that authorities stop counting mail-in and absentee ballots. He’ll have teams of lawyers challenging counts and ballots across the country.
> He also seems to be counting on having the advantage of mail slowdowns, engineered by the recently installed Postmaster General Louis DeJoy. Fewer pickups and deliveries could mean more late-arriving ballots and a better shot at dismissing votes before they’re even opened, especially if the campaign has successfully sued to block states from extending deadlines. We might even see a Brooks Brothers riot or two, where well-heeled Republican operatives stage angry and voluble protests against ballot counts and recounts.
> If Trump is leading on election night, in other words, there’s a good chance he’ll try to disrupt and delegitimize the counting process. That way, if Joe Biden pulls ahead in the days (or weeks) after voting ends — if we experience a “blue shift” like the one in 2018, in which the Democratic majority in the House grew as votes came in — the president will have given himself grounds to reject the outcome as “fake news.”
> The only way to prevent this scenario, or at least, rob it of the oxygen it needs to burn, is to deliver an election night lead to Biden. This means voting in person. No, not everyone will be able to do that. But if you plan to vote against Trump and can take appropriate precautions, then some kind of hand delivery — going to the polls or bringing your mail-in ballot to a “drop box” — will be the best way to protect your vote from the president’s concerted attempt to undermine the election for his benefit.