It is important to check email addresses and settings. I sent an email to someone once
and they called later that day screaming all kinds of things and had sent me a pile of nasty and threatening emails because they had received thousands
When we took a look it was pointed out that they all had the same timestamp on them which meant it was only sent once. It turned out that he had set one email address to send a copy to another email address which sent a copy back to the first one, - again and again ... and again.
I'm still waiting for that apology.