6-line addresses (or rather 7-line, including the name of the addressee) do occur.
Consider for example a typical UK residential address:
Miss S Pollard name
1 Chapel Hill building number and street name
BOURNEMOUTH town with a post office
BH1 1AA postcode
UNITED KINGDOM country
If the delivery was to a place of business, the address would need an extra line for the name of the business. I've known places with an institution name, a building name, and a street address, plus the town, postcode and country make 6. Some addresses may need a c/o line. That could reach to two more lines than fit on this form.
While I'm at it, here are a few things about US addresses that differ from a UK address:
- The postcode contains letters as well as digits. It can be 5 to 7 characters long.
- The postcode is below the city name, not on the same line. (Mixing that one up is unlikely to foul up the delivery though.)
- There is no such thing as a state or province. (Some sites have lost sales to me because I gave up trying to work around their “state field must not be empty or contain only whitespace or contain
_ or …” restriction.)
Wikipedia has a list of address formats by country (currently covering only about 1 country in 4). There is considerable variation. The UK example is by no means an outlier in terms of address size. As we've seen above, 7 address lines cover most cases but can be restrictive, though I expect that in practice most addresses can fit with some harmless rearranging.
For extra fun, some countries go from most significant to less significant element, i.e. put the country name at the top and the name of the person at the bottom.
Most post office administrations will work hard to deliver all letters, even with incomplete addresses, though it can sometimes take weeks, and there is a risk of loss if a key element is left out. Parcel deliveries are more iffy: often, if the routing software can't work out where the destination is (a country and postcode is usually enough at that stage) or if the delivery truck can't find the right building, the parcel will be returned. Worse, if the delivery agent can't find the right person, the parcel may be delivered to another person who may not always be trusted to deliver it.
All in all, you should be very liberal when allowing people to write their address in the right format. People know their own address and their local address formatting conventions better than you do.
There's one exception to that rule: for international delivery, the name of the country should be written in French or English (the official and working languages of the Universal Postal Union), and in practice it is probably safest to use English plus the name in the sending country's main language (e.g. “ALEMANIA / GERMANY” if writing from Italy to Germany). This is one area where you as the sender and professional know best, and you should let the user select from a list. Just make sure to keep your list up-to-date.