With six days left to go until Election Day, officials are encouraging voters who still want to cast a ballot from home to avoid using the U.S. Postal Service to return their ballot.

The warning comes amid concerns over the Postal Service’s ability to deliver ballots on time to be counted, especially in an election where tens of millions of voters nationwide are choosing to vote by mail instead of in person.

On Wednesday, the Maryland State Board of Elections urged voters to use official drop boxes to return their ballots from this point on, even though state law allows ballots that are mailed in to be received as late as Nov. 13 to be counted, provided it was postmarked by Election Day.

“While we don’t anticipate any issue with ballots already cast by mail, we are closely monitoring reports from the United States Postal Service that delivery times continue to be considerably longer than normal,” said Maryland State Board of Elections Administrator Linda Lamone in a statement. “In light of this, as we approach Election Day, we encourage voters to drop off their ballot at an authorized drop box. This will allow them to be confident their vote will be received and counted in a timely fashion.”

Almost 1.7 million Marylanders requested mail ballots for the November election; more than 1.1 million have already been received.

D.C. has the same Nov. 13 deadline for the receipt of mailed ballots that are postmarked by Election Day, but the city also has 55 ballot drop boxes voters can use. (Voters can also drop off ballots at early voting sites, or at polling places on Election Day.) As of Monday, 146,192 voters had returned their ballots, with more than 96,000 of those choosing to use the drop boxes instead of the mail.

“Remember, DC law gives the Board of Elections *10* days to receive & count your ballot if postmarked by Nov 3. We extended expecting USPS delays. That said, with delays & Election Day near, might be time to shift your voting plan to dropbox or VoteCenters,” tweeted Ward 6 Councilmember Charles Allen.

In Virginia, the deadline for election boards to receive mailed ballots is tighter: Nov. 6. During a special legislative session in Richmond this summer, lawmakers approved a bill allowing localities to set up drop boxes. More than 1.1 million Virginians requested absentee ballots for the November election; 847,720 have been returned thus far.

Early voting is also ongoing across the Washington region. In Virginia, it wraps up on Saturday; Fairfax County has extended hours on Thursday and Friday to handle what’s been a crush of residents seeking to vote before Election Day. More than 1.4 million Virginians have voted early so far.

In Maryland and D.C., early voting continues through Monday. More than 451,000 Marylanders have voted early so far, as have 37,173 D.C. voters.