Sounds dumb, but envelope mistakes are among the most common reasons for rejected ballots!
First - Use Only the special return envelope that came with your ballot.
No other envelope will be accepted.
Now, if you live alone, and you've used the envelope provided, you can go on to the next way to avoid rejection . . .
BUT, if there's more than one voter in your home, everyone needs to know this:
Match the Ballot Code to the Envelope Code
Every mail-in ballot comes with a return envelope that has a code number on it that must match the number on ALL the matching ballot sheets.
Sometimes members of the same household accidentally mix up their election materials and return their ballot sheets in another household member’s envelope. That invalidates your ballot!
Double check to make sure the code number on EVERY ONE of your ballot sheets matches the code number on your return envelope.