<prompt> Human: I dont know how, but I omitted the full PO box number, using PO Box 770, instead of 7704. I made a mistake.It was sent registered mail and came back as undeliverable something, as the red stamp was smudged. Anyway, IRS didnt get my taxes or check. How screwed am I? Anyone have suggestions?  <response>: Id photocopy the envelope, and wrap the still-sealed old envelope in a new envelope and resend it certified, perhaps with a short cover letter. 