They are receiving mailed returns, but it won't be processed for a while since the bulk of the personnel are sheltering at home due to the Wuhan Coronavirus. See the quote from the IRS below.
If you mail, it's recommended to mail by a method that has tracking, such as certified return receipt, priority mail, etc., so you'll know they received it. That should also provide you proof of mailing and a date you mailed it.
The IRS website says:
"To protect the public and employees, and in compliance with orders of local health authorities around the country, certain IRS services such as live assistance on telephones, processing paper tax returns and responding to correspondence are extremely limited or suspended until further notice. All Taxpayer Assistance Centers remain temporarily closed as are many volunteer tax preparation sites until further notice."
"The IRS is not currently able to process individual paper tax returns. If you already have filed via paper but it has not yet been processed, do not file a second tax return or write to the IRS to inquire about the status of your return or your economic impact payment. Paper returns will be processed once processing centers are able to reopen."