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Probably depends on the ISP. For Yahoo, their Help says:
"When emails arrive from addresses that you’ve blocked, Yahoo! Mail automatically deletes them. The emails never make it to one of your mail folders, not even your Spam folder, and they aren’t bounced back to the sender." 
On Gmail it's:
"While you can't currently block messages from specific addresses or domains, you can set up a filter to send those unwanted messages directly to Trash." 
So in neither case does the sender get any idea you've blocked them.
It depends on what email account server they are using. Most of the time the email is just not sent at all. Their email tells them that it was sent but you will never receive it. Most likely the email is stored on a server somewhere but usually its gone forever!
If you have windows live mail you can bounce them back to sender as if you no longer exsisted
For every email that doesn't get received, the sender's computer gets one virus. If they start to send you a lot of emails, they will get their computer screwed over.
They are returned. They will not know they are blocked but could guess as much.
it gets answered "undeliverable."
3 answers hidden
If I block emails from somebody, what happens to the emails they send?
Are they returned - does the sender know their emails are being blocked?
- Why did staff have to change email layout? Is there a way to get old one back?
- All emails have vanished and cannot retrieve them, even though i have signed in as normal?
- What's the point of Smart Views?
- I deleted an email by accident and i have no idea how but my trash got emptied. is there any way I can get that email?