Trump is exactly right.
Mail in balloting is chock full of fraud.
You may want to look at what just happened in Paterson New Jersey as just one example. Blatant, well planned, brazen mail in voting fraud is what happened there. Over 20% of the votes were fraudulent. They just ordered the voting to be done again. And that's just one small town in New Jersey. Imagine the same time hing across the entire country.
The Dems have planned for MASSIVE cheating. They are not going to get away with it though.
Stymied by the standard protections that are in place almost everywhere.
No one says people won't always try to game the system - mail-in or not - but it isn't that hard to put security measures in place.