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Addressing envelope 101
Start here. This is the short list of things that decide whether a scanner can read your envelope at all, and the mistakes that trip up almost everyone at least once.
Questions people ask
Why did my letter come back with no reason written on it?
Most returns carry a small yellow sticker with a code on it. If there is no sticker, the usual cause is a missing unit number, since the carrier has no way to note which box was wrong.
How long should I wait before assuming a letter is lost?
Give First Class mail ten business days before you treat it as lost. A wrong ZIP can add another week on top of that because the letter travels to the wrong plant first.