The US Postal Service screwed up. Take the envelope back to the post office and mail it again.
IRS Form 8453 - https://www.irs.gov/pub/irs-pdf/f8453.pdf
When and Where To File
Mail Form 8453 to:
Internal Revenue Service
Attn: Shipping and Receiving, 0254
Receipt and Control Branch
Austin, TX 73344-0254
