How can I found out who sent me an anonymous nasty e-mail?

Answer 1

open that email
if you don't have the "long header" option set - go into the Options section of your email program and check off - long headers
this will open all the information on who sent the email
write down the IP address
then go to the melissa data website - to the Lookup section
http://www.melissadata.com/lookups/index.htm

then scroll down to IP locator
then type in the IP address

once you have found this information you can also report it to your email program - abuse@ name of email.com
example - [e-mail]

Answer 2

Probably too late for this one, but for the future:

In Yahoo Mail, cl Options, General Preferences
Scroll down and put a checkmark to "display full headers." You will get information on the sender and can trace or report.