I want to be able to reject emails from a specific email address and
have it returned to sender, not sent to Junk E-mail.
Not built into the product.
I'd first look at any options your ISP might provide you. If there isn't,
you could dig through
http://www.slipstick.com and see if there are any add-ons.
If you're comfortable with code, you could build your own working with
http://www.outlookcode.com
for assistance.
However, serverside is the best option. If it's client side it only happens
when you download mail and, if I sent someone a message and didn't get a
failed message for 6 hours (because your machine was off), I wouldn't assume
it was because the address didn't actually exist.