Some email programs let you change your email address and still send messages, so that your email looks like it came from another address. You can do this by changing your address in your application configuration settings. Of course, replies to any email you send will then go to that email address and not to your real address.
Changing your email address is considered to be unethical if you are just pretending to be someone else, or if you are sending unwanted email under an assumed name. However, a common and ethical reason you might want to change your address is when you are using an application that combines email and newsgroup capabilities; if you are posting messages to the Usenet newsgroups, you may want to disguise your email address so that email spammers won’t pick up your real email address. Other times, you may have more than one address configured for difference purposes that are all forwarded to a main address, and sometimes wish to send email under one of the other addresses, for example when sending to mailing list configuration software.
If you ever find while surfing the Internet that someone has assumed your email identity to send email or post to the newsgroups, you should be able to get the help of the organizations involved to trace the fake messages back to their sender through unique usenet Message-ID numbers and traces of their IP address. However, this sort of problem is extremely rare.