Q. "I found out from returned "undelivered" email that those address does not exist in my lists--in the first place, I have never sent them".

A. Many affected owners of email domain being spoofed are complaining of having their account being suspended for spamming and receiving "tons" of harassing hate mails marked as coming from them--which they have never sent and only found-out when they start receiving angry replies or returned undelivered bounce email.

The victims of these new from of harassment in which fake or boogie messages was sent-out posing as the original owner of the email address by spoofing, are usually done by dissatisfied or fired employees, competitors, pranksters, junk mailers, provocateurs or spammers trying to sell something--not that they want you to respond to the email but to click on a link in the message.

According to the FBI, spoofing is generally not illegal because no hacking is required, unless it involves a direct threat of violence or death and by using such tactic, know as email spoofing--they exploit the simplicity of Internet SMTP (simple mail transport protocol RFC 821).

Email can be spoofed by tweaking the settings on standard email client like; Eudora, Outlook Express, etc.

There are also many website that offers an automated process for creating and sending spoofed email by inserting someone else email address into the Mail FROM: or REPLY TO: fields--which also contains information about the "origin" of the message--but most people don't know how to decipher it or simply assume that the spoofed message is genuine.

Spoofing is usually to obtain info, sell something, computer infected by spam zoombie, spambot or simply hate mail by assuming another's ID and making the recipient think that the email is from the sender...

a) Pretending to be a legitimate bank, etc., to get your ID.

b) A link in the message body taking you to a sales site

c) Attachments with virus or hate message

d) Selling something by using an infected computer to send-out sales pitch (unknown to the owner of the machine) and at the same time "spoofing" the spam using the computer owner address book email list.

If your email got bounced back to you with contents of: (a) or (b) then you know that someone machine got hijacked by spam zombies, sending out sales messages spoofed with your email address.

Usually, not only your email address but others as well from the address book of the hijacked machine.

If it is (C) it is usually a hijacked machine trying to propagate virus, perhaps someone sending it for fun or someone (perhaps an ex-employee) trying to create problem.

Spoofing does not mean that your computer is infected by "worm, virus, spambot, zombies, etc." and sending out messages from your address book.

It could be that "someone or your friend" machine (containing your email address in the address book) is infected or hijacked and sending out messages with your email as well as others, randomly taken from the infected machine address book.

Normally, tracking or finding the culprits is not that difficult,
because Deciphering Fake Email; every single Internet email message headers contains...
1. An origin (the machine that sent it)

2. Relay (the machine relaying it to another machine)

3. Final destination (the machine that receive it)

4. IP address  and  5. Domain name

By using tools like nslookup a DNS whois tool that perform forward and reverse DNS queries for the current domain address (to get an IP address of a hostname and hostname of the IP address).

See: Tracking Email - Can bulk or mass email marketers overcome the problem of mail server Port 25 blocking or mail servers spam filters by using faked or forged email headers, IP's or email domain with no MX Records?

See: My email was hijacked / spoofed and got blacklisted?

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