Thread by @patio11

Me to financial firm: *address change form*
Financial firm: Is this five digit number a post code?
Me to financial firm: Oh you have asked exactly the right person for geeking out about post codes. Did you know...
Second thoughts: That was not the efficient way to answer.

"Why didn't they know what a post code looks like?"

Because a post code can look like so many things, like 100-0001, 20500, or SW1A 1AA, to use three codes from three nations that all correspond to a particular famous building/complex within them.

A further fun fact: some nations don't customarily use post codes and others don't customarily use addresses, favoring a natural language description of the recipient which is sufficient to get a mail carrier to successfully route to them.

From these charming cultural differences come many software bugs, like e.g. US- or Japan-based programmers assuming that two computers referring to the same post code will pick the same casing for it, because of course numerals don't change if e.g. you toLower() on them.

And thus if you receive two equivalent post codes from two different computer systems you don't have to normalize casing before comparing them because, hah, why would you need to do that.

Also while I'm at it: you might think that most post offices who customarily use post codes really need those post codes, but you might be surprised at how much effort they put into making them optional but highly preferred.

In the US, one of the USPS's backups to missing post codes is a collection of buildings where clerks look at OCRed images of automatically sorted mail in substantially real time with a keyboard designed to optimize for routing it with as few keystokes as possible.

I am not aware of the existence of a similar system in Japan, but am aware of a fallback which once made "Patrick McKenzie, Ogaki, Japan" fully routable.

That system was "ask around the Ogaki Post Office" until someone handwrites 'Probably the American who lives on (internal identifier for route) near the river. Works salaryman hours speaks Japanese. Confirm before delivery.' on it."

"Who tried to send you a piece of mail with that 'address' on it?"

Social Security Administration failmerged an account statement.