If you can show me solid evidence that, say...2500 mail-in ballots were fraudulently filled out and submitted in a given locality or state, I'll definitely believe it's an issue.
I don't think that evidence exists or will exist.
There is no way to know that until after the election - if ever. What can be shown is dozens and dozens of instances of mail ballots being thrown away in this election that has been discovered. In the Democratic Primary the totals known are well over 100,000, I think that even in just one state. But I get tired of looking up the same stuff over and over again - since there is no reason.
I was an election judge in a Democratic Primary under the old system. What surprised me was how easily I could have cast hundreds or more ballots in a way I could not be caught - and even if I was the votes would still count since there is no way to know which votes to eliminate. But I had no incentive to do so.
Election divisions do have incentive. Those are partisan political positions. For they and their management staff their side has to win the election - or they are out of a job. If they want a promotion within that party structure, your precinct or county hugely winning is a plus - and we see how fired up people are. Are people who riot, protest, do car and boat parades - willing to merely fill out ballots so their side wins - with they all thinking it is the end of the world if their side doesn't?
Explain to me what stops election fraud? It now has been ruled it does not matter if the ballot is signed or even if the name signed matches the voter's name. Opposing party poll watchers are NOT allowed to see mail in ballots, NOT allowed to challenge any mail in ballot, and not able to watch the processing or tabulating of the mail in ballots - and anyone can just photocopy them in most stated, not even having a watermark. In many key states, mail in ballots can be received and counted as much as over 2 weeks later, if postmarked by the deadline. Anyone could have a postmark stamp made for about $100 or make it themselves.
I also know from my other political involvement, that every partisan precinct judge has a list of registered. The partisan division has a list of all registered voters - and will know who didn't vote - meaning available. Usually that is 40% to 50% of registered voters. Even if a duplicate to someone who did vote, it just would mean that one wouldn't work. In addition, even if they learned a country election division person or entire department cast 200,000 fraudulent ballots, there would be NO WAY to remove those fraudulent votes from the total since no record may be kept of how a person voted. And they would then know how many votes they need to add to win the state, Congressional office or any local election.
NOW... with that system, explain ANYTHING that prevents massive numbers of fraudulent mail in ballots - and if learned this happened all those votes still count.
I've posted long before this election and without partisan finger pointing, the system of voting is DESIGNED for voter fraud because BOTH parties what to keep the majority where they are - and the majority controls. However, it was never before THIS easy on an industrial scale.
If they wanted to end this, all it ever would have taken is requiring a fingerprint. This has been known for decades. But no one REALLY wants honest elections in partisan jobs - particularly when keeping their job is dependent on winning the election. So nobody in power at any level wants to stop voter fraud. Rather, they want to be the side in control of it.