Email Spoofing:

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The practice of forging email so that it appears that the mail was sent by you, implicating you as the originator of the spam when this is not the case.

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example: Your email address is user@yourcompany.com and you have received a spam email from randomaddress@yourcompany.com.

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Spoofing is used to trick the recipient into revealing personal information; suggesting that the user reveal passwords, bank-related information or other sensitive infomation.

Spammers use email spoofing as a way to hide their true identity, in this case their IP or email address. Spoofed emails often contain a web-link embedded in the email that when clicked may dowload a trojan into the users machine or send the user to a  malicious website.

Self-Spammed Spoof Emails:

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Self-sending spam is unsolicited e-mail that looks like you sent it to yourself. The 'From' address is similar or exactly the same as your own email address.  Your name may even appear on the 'From' and 'To' lines.

Autoresponders:
Autoresponders are especially susceptible to self-spam based emails. When one of these self-sending spam messages is sent to an autoresponder, a message is returned to the sender, which, in this case, is the autoresponder address.

Thus depending on how many spoofed emails are received within a given time frame two emails will be in the Inbox; the original spoofed email and the auto-responder's response.

Bounced Emails:
Bounced 'spoofed' emails look as though they were sent from your email address to an unknown address or non-existant address, then the email bounces back to your Inbox. 

These 'bounced' emails are spam; never respond to these emails, either mark them as spam or delete them. Acknowledging them by responding lets the spammer know that your email address is a valid one and the rate at which you are spammed will most likely increase.

Unfortunately, there currently is no way to prevent receiving a spoofed email. The best current practice is to mark the email as spam or delete it.  

Remember that although your email address may have been spoofed this does not mean that someone has gained access to your account or password.