The President sent out this email and let's try to follow his logic.

Imagine you are intent on using mail in voting to commit voter fraud.

First you have to know what day your neighbors mail-in-ballot is going to be delivered.

Then you have to get to the mailbox and steal the ballot before they get there

Then you have to hope they don't request another ballot, thereby invalidating the one you stole.

Then you have to fill out the ballot and forge their signature and hope that that signature looks enough like the real thing that it doesn't get tossed by an election judge

And you have to hope the person you stole from does not vote in person, which will tip them off

And you have to do all of this in the hopes of changing just one vote.

One. Vote.

Now imagine trying to scale this up. Imagine trying to do this the thousands of times you would need to inorder to impact an election without getting caught.

Voter fraud is so easy to catch and offers so little benefit that it just isn't a thing.

It is like trying to pass off a trip to Burger King as a healthcare expense on your taxes. The risk of getting caught far outweighs any small benefit the lie provides.