If you file your tax return and then the IRS loses it and sends you a letter saying you never filed, how can you prove you did? You may think the IRS does not lose tax returns, but they do every year. There are five primary ways to file that the IRS recognizes as providing proof of time of filing. 1. USPS Registered mail – You must keep your receipt to prove time of mailing. 2. USPS Certified mail – You must keep the green card proving time of delivery. 3. IRS approved e-filing – You must retain your electronic postmark as proof of filing. 4. IRS approved Private Delivery Services such as DHL Express Same-Day, Fed Ex Overnight, UPS Next Day or Second Day services. Other services from these Private carriers are not acceptable. 5. USPS Regular First Class mail. You must make a copy of your envelope showing postmark, but this is the least reliable method.