It's not a concern at all. Colorado has had mail-in voting for years by default, with the option to show up at a precinct. Every ballot is bar coded. I get an email when it's mailed. I get an email soon after I've dropped it off at an official drop box.
Every current and past Secretary of State from each state will tell you election fraud happens, and that it's rare enough it doesn't have an effect on the outcome.
Trump is lying when he said there is a 100% certainty of a rigged election if there's widespread mail-in ballots. Be clear about what he means by rigged. A system-wide fraud that influences the outcome of an election.
It's the same kind of lie about 3 million voters being "illegals" in 2016 and why he lost the popular vote. It's the same kind of lie he told about buses being shipped up from Massachusetts to New Hampshire to explain why he lost New Hampshire. The same lies about "you will not believe what my people are finding in Hawaii" about Obama's birth certificate. And the thousands of people cheering on 9/11. And the hundreds of people he knew who died on 9/11 yet went to no funerals, zero zip.
And it's the same tactic he used in 2016 to set the stage for his loss. When asked if he would accept election results if he lost he refused to say yes, he only said he'd accept the election results if he won.
He excels at creating doubt and confusion. That's his entire life history way before he was in politics.
He's an asshole. He's a complete waste of space. He's a whiny little bitch. He's always been this way. It's not new. He was this way when he was a Democrat too. As president. As candidate. Before he was even in politics. He has always been a piece of shit asshole. He will always be a piece of shit asshole. And hilariously this is a completely unremarkable observation. The absurd claim would be that he's a compassionate person of strong ethical and moral character, a role model you want your kids to look up to, mimic, and be like when they grow up.