[NonValueAdded]

It speeds up the sorting process at the receiving end. If the street address has close duplicates (e.g. Smith St. Smith Ave. Smith Rd.), it might lessen the chances of a sorting error, putting the letter with the wrong route carrier.

[kevkrom]

I've never noticed any significant difference. I would imagine that using a correct 9-digit ZIP would get it there faster during heavy workload periods (less manual sorting), but there would be no guarantees.

[IllumiNaughtyByNature]

It may only help during Tax Season and Christmas when postal volume is at it's peaks. That's about it and we mail about 30K pieces a month.

[Mad Dawg]

I actually went postal, uh, well, no, for about 18 months in my life of uselessness. In most cases, the 5 digits are about getting it to the PO in question. The Plus 4 are about what happens to it at the local PO.So the benefits of the Plus 4 are going to be more efficient use of labor and have more to do with costs than with speed in any way that would be obvious to the cussamer. The ideal bottom line of an address is with NO PUNCTUATION CITY TWO_LETTER_STATE_CODE ZIP_PLUS_4 If you print it clearly then machines can read it and that really will make a difference in speed. If the scanner can't figure it out, some poor schlub who remotely (like a couple of states away, maybe) sees what the scanner sees is invited to take a stab at it - which slows things down for everybody. If he/she can't dope it out it gets spat out and figured out later. My recollection of PO work is that it totally stinks. Be nice to your letter carrier, s/he's having a bad day.

[HeadOn]

Basically, the above posts are right. If there is sorting equipment at the destination Post Office, it helps them with automated sorting. And - since individuals very seldom have access to, and are not required to use all 9, I would hazard a guess that mass-mailers with computerized lists ARE either required to use, or receive a deeper postage discount for using the full zip+4.

The first five digits are the Post Office itself, or in some cases, a large corporation or university may have a dedicated zip.

If you ever see a P.O. Box address with 9 digits, the last 4 are either the P.O. Box, or obviously related to it.

[Dont Mention the War]

I think the easiest way to respond is: It won't hurt, but it may or may not help, particularly if you're hand-writing the address.