Some states in the U.S. conduct All Mail Elections but those are not mandatory.
Due to the COVID-19 pandemic, more states have planned mail-in voting to prevent large crowds from gathering at polling places. Voting by Mail is also an essential part of the elections process for citizens away from homes, such as military and overseas voters. The authorities are working together to ensure the entire voting by mail/absentee voting process works smoothly and efficiently.
At present five states currently conduct elections entirely by Mail: Colorado, Hawaii, Oregon, Washington, and Utah wherein the registered voters receive a ballot in the mail. But it does not preclude in-person voting opportunities on and before Election Day. The voters can choose to cast a ballot at an in-person vote centre during the early voting period or on Election Day.