Voters in Pennsylvania’s May 16 primary election will choose their parties’ statewide candidates for the Commonwealth, Superior, and Supreme Courts, plus a slew of candidates in local races for mayor, council, school boards, and more.

All of these voters will have a chance to cast their ballots by mail. But thanks to Pennsylvania’s patchwork of mail voting regulations, their experiences may differ slightly county to county.

The commonwealth dramatically expanded mail voting in 2019, allowing all registered voters to request mail ballots without an excuse. But since then, mail voting h...