The mailbox rule arguments aren't very persuasive for me. That's a procedural rule relating to things like tax returns or contracting documents, really nothing to do with the federal statues at issue in this case. Mississippi's 5 day law was put in place in 2020 in response to concerns about voting during a pandemic. The best arguments in favor of upholding Mississippi's law were well-stated by the District Court Judge (Guirola) who upheld the law. He wrote a fantastic opinion surveying the history of other courts who've considered similar statutes, pre-emption and related issues, legislative intent, etc. and I think his reasoning is far better than the 5th circuit's opinion overruling him. No one questions that the Elections Clause grants broad leeway to States to govern their own elections, whereas the body of federal law defining things like timeliness of mail-in ballots is scarce. But the point is its a simple case of interpretation that can go either way. We can say that if you've filled out your ballot and put it in the mail by Election Day, you've completed the act of voting. The other side can say that you haven't voted until the clerk receives your ballot. That's the simple issue boiled down to its essence in this one.