First check that this is a real warning. Check for the usual signs of a fraudulent email (false address "from", which links you to a website that does not end with .gov). Contact U.S. immigration on an email or publicly available number (not through a contact you received in the warning) and check with them.
If it turns out to be authentic, it's probably not as bad as you think. It is most likely an administrative mix.
When you contact immigration, they will tell you what to do, but most likely if you just give them the date and flight number when you depart, it will be the end. They will almost certainly be able to verify that you were on this flight, and that will likely be the end of the problem.
Also remember that, even if for some reason they do not believe that you have left on the flight you have flown, you do not need to prove that you have departed on that flight, it will it is enough to prove that you left before the end of your legal stay. in ten days. The easiest way to do this is probably to pay some sort of visit to an official, such as a government or police office or a lawyer, where your visit will be recorded and you will be able to provide information. But something as simple as recording a daily transaction will help – some records of you using your credit card in your hometown are going to be proof and are probably pretty good.