Postal ballot fraud is a conspiracy theory
Voters are far more likely to make mistakes when they cast postal ballots. Postal ballots may be rejected because inexperienced voters make technical errors. For example, in New York,sealing a postal ballot envelope with tape is grounds for disqualification.
The Trump campaign has often claimed that increased postal voting due to the coronavirus pandemic will lead to tremendous electoral fraud. His campaign has claimed that voters may receive inaccurate absentee ballots (ballots for those voting outside of their state). Trump may be able to declare victory on election night, based on the tally of in-person voting while many postal ballots would still need to be counted in key states. The election could in fact be rigged for Trump as the public may be confused about the election, and Trump may be able to challenge postal ballots (and even throw some out in key states).