It has everything to do with PO boxes. UPS and Fedex or Dhl cannot deliver to PO boxes. The IRS has a specific list of street addresses (no PO boxes) for private delivery services:
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/filing/submission-processing-center-street-addresses-for-private-delivery-servic...>
I think this (which I found in the meanwhile) partially answer my own question. But I am having trouble finding the analogous information for Illinois.
I asked because I am mailing internationally.
