The IRS does not have the burden of proof. You do. If the IRS says the extension was filed late, it's up to you to prove it wasn't.The IRS goes by the postmark on the envelope and enters that date into your account. They may mis-key the date, generating a late notice when you file your return. You need to prove they made a mistake.If you mail an extension, send it by certified mail or some other method that gives you proof of mailing and preferably proof of delivery. Don't wait until the last day. Odds are you know you need an extension well before the deadline.The best way though is to file the extension electronically. You will receive proof of sending and proof of receipt.