You have to register to get an absentee ballot. A mail in ballots is where you receive a ballot at your last known address they have on you. These are not the same thing.
Change of address, similar names, postal issues can all affect the receipt of your ballot. This is a big problem when you're sending out 100 million of them.
I frequently get federal letters for the last occupant at my house and sometimes my letters are lost, sent out to someone with the same name or my old address. If this was a mail-in-ballot then either I wouldn't get a vote or someone else could vote in my name.
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