State and local election officials from across the country on Wednesday warned that problems with the nation’s mail delivery system threaten to disenfranchise voters in the upcoming presidential election, telling the head of the U.S. Postal Service that it hasn’t fixed persistent deficiencies.

In an alarming letter, the officials said that over the past year, including the just-concluded primary season, mailed ballots that were postmarked on time were received by local election offices days after the deadline to be counted. They also noted that properly addressed election mail was being returned to them as undeliverable, a problem that could automatically send voters to inactive status through no fault of their own, potentially creating chaos when those voters show up to cast a ballot.