Emails can't be rewritten by anyone except your mail provider, which is unlikely to ever happen.

What most likely really happened is that the mail included remote content which got reloaded - and thus changed.

Webmail provider*

Sorry for being pedantic, but if you sync your mail to your own device (or collect all of it onto your own mail server), you eliminate this risk.

The message you were sent probably just embedded a webpage. Still creepy as fuck, but explainable.

How is an email that loads remote assets "creepy as fuck"? It actually sounds useful in this case, all he had to do is look at an email to see what was going on with his flight. It doesn't have to be sophisticated, imagine an HTML email loading an image through a URL like so:

  /asiana/status?flight=214&date=2017-03-17

Pretty simple to return a standard "On Time", "Delayed" or "Cancelled" asset.

In part because it means that a property you take for granted in email (namely, that you can reference it later without it having changed) is violated. (This would permit all sorts of things involving documents being retroactively edited to whatever is convenient for the party sending the email.)

In part because you could use this to implement detection of an email having been "read" when the remote asset is fetched.

(These are just why I find those sorts of emails irritating and sometimes creepy.)

If you don't mind getting tracked, then you probably won't find it creepy.

I, on the other hand, get annoying letters form my bank saying I never open their emails, so they won't let me password reset via email anymore. Uh, OK, I'll waste the bank's money calling, I suppose...

...Well, because UI sucks, and email contents aren't supposed to change. If there's a small box saying "Current status of your flight", that would be one thing, but the whole contents of the email can change and (unless I check the raw HTML of every email) there's no indication which part is immutable. And I consider myself a computer-literate person. Mostly.

Imagine you book a hotel online, fly to another country, arrive at the hotel. "I have a reservation." "May I check your reservation?" "Sure, just a second." You take out your iPhone and open the email: but while you were flying the online booking site had a server meltdown and the data was corrupted.

"Uh, sir, the email says 'Unfortunately, there is no room available at this hotel now. Would you like to check other hotels in the area?' And it's dated three months ago."

"What the..."

I now always print flight/hotel reservations upon booking. That will be safe until nanomachine overlords arrive, I guess. Low-tech solutions FTW.

That shouldn't be possible. I wonder if they had put the message in there via an image on their site?