Here in Florida we've had fraud peristently over many years of a type that is only possible with vote-by-mail. Here it is known as ballot brokers and what ballot brokers do is find people whom they can "help" to vote, especially elderly people in facilities who are more vulnerable to being tricked. The ballot broker is working for a particular candidate and finds ways to collect a large number of ballots with votes for that candidate when the voter didn't know who they voted for and/or thought they voted for someone else. This type of fraud is only possible with vote-by-mail.
Another problem with vote-by-mail is that the chain of custody of the ballots is difficult to maintain. If someone wanted to disappear some ballots at a point early in the chain, it would be difficult to prevent it.
One good thing about vote-by-mail is that obviously it means that votes are on paper ballots. But here in Florida our laws say that all voting is by paper ballot regardless of whether cast through the mail or in person.
Under the assumption that all voting would be by paper ballot anyway, vote-by-mail doesn't offer any advantage with regard to fraud, it only brings some additional fraud opportunities.
The real solution to election fraud is a completely transparent election, with paper ballots deposited into a transparent ballot box (so observers can see it is empty when voting begins) and then a transparent hand count on the spot before ballots have been transported or stored. This type of system is used in the UK, I believe.
Unfortunately, the complexity of our political system makes counting by hand impractical so the solution, as if it weren't hard enough already to get enacted, would include a reorganization of the way we divide up our political system and the offices that follow from it.
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