Election fraud involving mail-in ballots is very rare. Currently, about one in four Americans votes by mail in federal elections and it is also the primary method of voting in five states (Colorado, Hawaii, Oregon, Utah and Washington).
It is true that mail-in voting is more susceptible to fraud than in-person voting and ballots may be forged, intercepted in the mail or harvested in favor of one candidate. However, as the Brennan Center points out, states can easily enhance election security, using strategies such as identity verification, ballot tracking and severe criminal or civil penalties.