When sending letter size packets to the US, I sometimes get them returned as "address does not exist". After some discussion with customers and their local postal service, it appears that USPS ONLY reads the last 4 lines of the address.

Therefore if the first line of the address goes over 2 lines, their automated systems cannot find the full address (as it is now 5 lines long) and simply return the post!

This does not happen on Large Letter / Small Packet as the font appears to be smaller.