Assuming you are serious, the story is as follows:
Mail-in voting is fraught with opportunities for fraud. Generic ballots (ballots with no unique identifier) are mailed out to every name and address that the voting machine can locate. This includes people who have died, people who have moved, people who have not voted in many, many years. Usually mail in ballots number much more than the voting population. And of course the political parties are ALSO sending out mail-in ballots to their constituents.
Contrast this with "Absentee Ballots," where an individual, registered voter formally requests a ballot from the election authorities because for one reason or another they will not be able to vote in person (working out of state, can't get off work, etc.). These ballots only go out to legally registered voters, and they are uniquely identified with that voter. Usually a witness and a signature are on the Absentee Ballot, so there is little room for fraud.
So what could happen with Mail-in Ballots? People could just fill them out and mail them in with the names of random other people. Political operatives can determine which voters rarely vote, and submit Mail-ins for those people. If it turns out to be a duplicate, who cares? The sender cannot be identified. The election authorities in most cases are told that irregular signatures are NOT a reason to disqualify a ballot, which makes it even easier to commit fraud.
So the question is, if there were some political party or organization that WANTED TO TAKE ADVANTAGE of the much-too-loose controls on Mail-in voting, would they be able to do that? The answer is manifestly, yes.
Then you look at precincts in large inner cities (e.g., Philadelphia) where 100% of the Mail-in ballots are straight-ticket Democrat votes, and unless you are either stupid or compromised yourself, you think, There's something fishy going on here. Black males are voting Republican nationally at a rate of 15-20%, and yet in Philadelphia it is ZERO PERCENT? What. The. Fuck?