If you remember when you setup your email client you were able to have your name show up any way you liked? Well entering the "reply-to" email address is a similar field which is optional to the sender... in other words, anything can be put there.
There is a proper way to read SPAM mail... this is done by viewing "full header" information. If you are using Yahoo you can turn on "full headers" by clicking on OPTIONS in the upper right of your screen when in the email page... then going into General Preferences > Messages > show all headers on incoming messages >> save
Now you will see all of the gobbledygook above the message body... stuff like x-originating ip and received by ip etc... etc...
Some of this can also be faked or "spoofed". However it is much harder to spoof this information.
If you are wanting to report spam you should always forward it to the authorities with the full header information. Sometimes you will have to manually copy and paste in that info.
You can forward it to the Federal Trade Commission.. they handle spam, phishing, internet fraud, etc... and their email address is spam@uce.gov
If there is a valid yahoo address sent to
network-abuse@cc.yahoo-inc.com
or abuse@hotmail.com
abuse@gmail.com
etc...
If you know how to perform a WHOIS lookup on IP addresses you can forward the email also to the ISP where it is truly originating from.
The main thing to remember is to NEVER click on the links in the email. !!!
If you hover your mouse over the links or images you will probably notice that the ACTUAL link it will be going to is quite different than you expected.
If you are interested in playing a really cute game from Carnegie Mellon University that will teach you all about Phishing check out this link:
http://cups.cs.cmu.edu/antiphishing_phil/quiz/index.html
I strongly urge you to invest in a proper security suite from a reliable vendor like McAfee which will protect you from accidental clicks and malicious attacks.
Good Luck!