The Inspection Service is working to protect the vote
The Postal Inspection Service is the federal law enforcement agency assigned to protect the mail including Election Mail and Ballot Mail.
On its website, the agency outlines its efforts to protect the integrity of election-related mail as it moves through the postal network.

These efforts include
The site also offers tips, including recommending customers pick up Election Mail as soon as it’s delivered to their mailbox, and drop off election-related mail inside a Post Office or a liteblue USPS collection box before the last pickup of the day.
To report law enforcement matters regarding election-related mail, customers and employees should call the Inspection Service at 877-876-2455 (select option 4) or submit a report online.