The address MUST be correct because the email arrived FROM there in the first place.
False. In most cases the return address has been typed in by the sender (somewhere when setting up the account in OE will do it for example). This is just as prone to errors as any other piece of typing. One
business I was trying to
buy something from had this wrong for weeks - it took me that long to persuade them there was a problem that needed fixing, and for them then to go and find whichever family member it was that had set the computer up in the first place to fix it.
The other, and more common, possibility is some sort of glitch at the receiving ISP. Try again later.