Absentee voting by mail/early voting is when a voter requests a ballot via mail. There is a record of your request with your signature and you mail it back in. Your signature is verified with your voter registration and then tallied. This is how absentee voting/early voting by mail has been done for probably 100 years. Trump has endorsed this as being legitimate.
Vote by mail is the real concern. This is when ballots are sent out with no control. This is when a household gets 5 ballots with only two people residing there since anyone who has lived there will also get a ballot - similar to junk mail. You can vote and just mail it in using a fake name or using your neighbor's name. Many states have said that they do not want to or can't verify these signatures since they don't have the manpower to verify all these ballots. This is where all the fraud will take place. It is going to lead to all kinds of chaos and the dems are hoping that if Trump wins, will cast the election as illegitimate. If a state is close, these ballots will all be suspect and the lawsuits and haggling over them will make Florida in 2000 appear to be child's play.
The only solution is for Trump's win be so profound, so deep, so big, that no one can call the election as fake and these mail in ballots become irrelevant. That is a tall order and probably won't happen, but we can hope.
I live in Oregon, a vote by mail state. They do not send ballots out to everyone. Our ballots are verified by signatures, if you forget to sign your return envelope it gets set aside and I’ve had my signature challenged and had to provide a new signature card.
Now you can even check online to see if your returned ballot has been received. I prefer the paper ballots of VBM to electronic voting machines in person.
Of course it has taken decades to fine tune it and I am deeply suspicious of the urban counties because they are always chaotic by dint of volume and the incompetence that government employment seems to encourage.
States that try to jump into VBM for a major general election are just going to end up with endless counting and litigation.