Why did my email apparently get delivered to the wrong person?

First, I have to say that all of these possibilities are unlikely. They're either difficult to have happen by accident, or they're likely to be things that you or your cousin would simply notice and fix.

But unlikely as they may be, clearly something's happening.

My first reaction is malware. Not necessarily on your machine (though of course you should check), but on your cousin's. It's possible that his email has been hijacked in some way so as to redirect what he's getting and sending it somewhere else.

So it would make sense that both of you should follow the traditional advice: make sure your anti-malware software is up to date and scan for both viruses and spyware.

My second thought, and the most common reason I send email to people accidentally, is my email program's "auto fill" feature. As I type the first few characters of a persons name or email address, the program automatically fills in either recently used email addresses, or email addresses from my contact list, so I can just hit enter after it hits the one I want, rather than typing in the entire email address. I'll end up getting use to typing, say "k", having it immediately bring up my wife's email address and I'll hit enter. That works fine until for some reason the list changes and some other email address beginning with "k" comes up first. All of a sudden I type "k" followed by Enter without paying attention and someone else gets some very embarrassing email.

You might check your sent mail folder to confirm that you actually sent the message to the email address you think you did. If not, I'd be suspicious of this kind of auto-fill misfire. (It doesn't always have to be the leading characters, by the way. I've seen email programs that try to be smarter and look for substrings, or check both the email address and the display name at the same time.)

You might also double check your address book. I've seen address book entries where the display name is actually an email address, but the email address associated with it is something else entirely. Who knows which one you'd be sending to.

My next thought was that your recipient has their email forwarded to another account. I have this setup myself: my Hotmail account actually forwards to my private account, which in turn these days forwards to my GMail account. Send me email on Hotmail, and I could read it, and respond from, any of at least three different accounts.

So, there's a chance that your cousin's email is being forwarded somewhere you're not expecting it.

Certainly there's a chance that the ISP screwed up, but that's well below the bottom of any list of possibilities I'd actually consider.

Yes, in theory, an email address - like a phone number - is exact. But, like a phone, sometimes things can happen that end up redirecting or hijacking it. Unlikely things, but things nonetheless.

Readers: if you've got an idea on what might have happened, post a comment; love to hear it.