Inspector General's report at https://www.treasury.gov/tigta/auditreports/2020reports/202046041fr.pdf

says on page 13:  "As of May 20, 2020, the IRS estimates that it has received 10.4 million pieces of mail while the Tax Processing Centers were closed. This includes mail stored within its various Tax Processing Centers and in 20 semi-trailers and six storage containers housed outside these Centers.  The unopened mail includes tax returns, taxpayer correspondence, payments, and mail returned as undeliverable. Each Tax Processing Center has a mail operation that opens, sorts, counts, and routes all incoming mail and packages to their appropriate destinations. These actions were all halted when the Tax Processing Centers were closed. Furthermore, all paper tax return processing was halted. "