Some states with mail-in voting allow so-called "ballot harvesting," when mail ballots are collected by third parties to be delivered to election officials. In general, signature verification requirements for mailed-in or dropped-off ballots help protect against any fraud or tampering with the ballots, whether through harvesting or other means. "Documented instances are rare," says MIT's Election Lab.
California relaxed limits on harvesting in 2016. Republicans complained about the boost it offered Democratic candidates in California in 2018, but, as the Federalist noted, there was "no evidence" of any foul play. In a 2018 congressional race in North Carolina, where the practice is restricted, a campaign operative collected and completed ballots in support of his candidate, resulting in a new election.