Why is everyone talking about vote-by-mail now? 

Elections are under the states' jurisdiction in the United States, so this varies from state to state. Some of them, like Colorado, Hawaii and the state of Washington, have been running primarily mail-in elections for years, so the majority of people there have experience with mailing in their ballots.

But in many other states, you needed to have a valid excuse to request an absentee ballot. In Connecticut, for example, you had to be traveling on the day of the election or be somehow incapacitated in order to be allowed to mail in your ballot. COVID-19 has changed all that. Now many states that required excuses before will accept "Because of the pandemic" as a valid reason to request a mail-in ballot.

"We are going to do mail-in voting," law professor William Dunlap, who is part of the coronavirus high-risk demographic because of his age, said about himself and his wife. "We don't want to go into a polling place where so many different people touch the voting machines. It doesn't feel safe." The 76-year old can avoid polling stations in November because his home state of Connecticut changed its rules. But senior citizens in states like Texas, New York or South Carolina still need a valid reason apart from coronavirus fears to be allowed to mail in their vote.

How does mail-in voting work?

Again, it's different from state to state. One major way in which the process varies depending on where you live is how the ballots are sent out. In some states that allow mail-in voting, every registered voter will automatically receive a ballot. In others, registered voters receive an application that they'll have to send back if they want to vote by mail.

Read more: Has US voter suppression become systematic?

To make it even more confusing, voting by mail in the US doesn't necessarily mean you don't leave your house to cast your ballot. Sure, one option is to fill out your ballot and then put it in your mailbox for the mailman to pick up, as is common for regular mail in the US. But you can also go out and put your ballot in one of the special mail-in ballot boxes that many towns across the US will set up in the run-up to November 3. That's particularly convenient for people who don't want to set foot in a crowded polling place, but don't trust the US Postal Service (USPS) to take care of their ballot.

Why would Americans not trust their postal service? 

There have been problems before — in the 2018 midterm election, thousands of filled-out absentee ballots that voters posted before the deadline were not delivered in time to be counted. This time USPS faces an even greater challenge as the number of people requesting a mail-in ballot is expected to increase exponentially because of the coronavirus. In addition, Postmaster General Louis DeJoy has implemented changes that are seen by many as weakening the postal service. DeJoy, a well-known ally of US President Donald Trump, reorganized USPS leadership and removed the top-two executives who were overseeing day-to-day operations. Before that, he had already banned employees from working overtime and making extra trips to deliver mail. Critics view these steps as sabotage.

"If the post office is run with the efficiency it's always run with, we should have no problems," Connecticut social worker Julie Strauss Carey said. "But there have been lots of changes and that's concerning. There are people who want our election to be chaotic."

Trump has publicly called the postal service "a joke." On Fox Business the president said this week that he opposes the Democrats' push for a funding increase as part of the next corona stimulus package. "They want $25 billion (€21 billion) for the post office," Trump said in a phone interview. "Now, they need that money in order to have the post office work so it can take all of these millions and millions of ballots."

Alluding to the difficult negotiations about the stimulus package, the president added: "If we don't make a deal, that means they don't get the money. That means they can't have universal mail-in voting." Universal mail-in voting would mean that every single registered voter in the entire US would be mailed a ballot automatically — something that isn't even on the table for this year's presidential election. But Trump has made it clear again that he opposes mail-in voting of any kind. 

Read more: Opinion: Trump ups the ante by suggesting election delay

Is there a risk of fraud with mail-in voting?

According to Trump, fraud is a huge concern with larger numbers of people wanting to vote by mail. The president has been railing against vote-by-mail on Twitter, claiming that people could steal ballots that are sent out to voters or print fake ones. But there is no proof that this has happened to any significant extent in the past. "I would not call voting by mail prone to fraud," said Charles Stewart, founding director of the Election Data and Science Lab at the Massachusetts Institute of Technology (MIT). "More people voting by mail is not going to lead to a more fraudulent or a less fair, less accurate election."

Experts believe one of the reasons Trump keeps bringing up the issue is that he wants to discredit the election results before a single vote has been cast. "He has done everything so that his supporters don't trust the election results," Sheri Berman, professor for political science at Barnard College, said. "His continuous talk about fraud will make it more likely for people to believe that something didn't go right if the results in November aren't to his liking."

What are the logistical challenges?

The ballots have to be printed on special paper (as a way to prevent fraud) and be mailed in special envelopes. The states need to hire more staff to open the returned envelopes, count the ballots and verify signatures, and in some cases even purchase special equipment. In some places, machines do the counting instead of humans. Then the new staff needs to be trained. All of that requires additional funds, at a time when communities and states are already cash-strapped because of the pandemic. "There are pockets in America that are not prepared" for a sharp increase in demand for mail-in ballots, Stewart says.

How long will it take to count the votes – and what would a delay in getting the results mean?

If Biden or Trump wins with a large margin, the final result will be clear on November 3, even if not all ballots have been counted. But the closer together Trump's and Biden's numbers are, the higher the chances are that the country won't know for sure who is going to be president on election night, or the next morning. Experts say that if it ends up being a neck-and-neck race, it could take weeks to get the final results. 

Stewart says that results from rural voting districts usually come in faster — which means that early on, Trump as the Republican candidate could likely appear in the lead. Later, when the votes from larger, urban districts come in, Biden could potentially be seen to catch up. In a situation like that, the political scientist says, Trump could call the results into question, especially if this is drawn out over days or even weeks.

That's a situation law professor William Dunlap is worried about. "It remains to be seen how his followers would take this," Dunlap said. "It could lead to mass resistance and violence in the streets."