When email is sent so that it appears to have originated from somewhere other than its true source. A common behavior for many viruses and spam systems active on the internet today.
A few situations which can commonly lead to increased spoofing:
- Your computer is invected with some type of malware. Make sure that all computers from which you access your email are clean. To do this make sure your current virus protection software is up-to-date and actively scanning your computer. To be safe consider running a full virus scan of your computer per the documentation provided by your virus protection software vendor.
- Some other computer is infected with malware. If your computer(s) do not appear to be contributing to the problem it is likely that there is nothing that you can do in this situation as the problem may be that someone else is infected with malware and just happens to have your email address on their computer (from previous communications with you or for some other reason), or simply be guessing your email address.
- If your email address is easily guessable, like "julie@mycompany.com" you are likely going to get a lot more spam and also have your email address more commonly be spoofed.
NOTE: A common result of having your email address spoofed can be to receive an email administrator message telling you that a message you sent was infected with a virus or was otherwise blocked. This often happens when a message is spoofed to be from you and then the server that receives the message rejects it. In this situation, since the recipient email server thought it came from you, you will unfortnately be the one to receive the response. If this happens all you can really do is delete the message.
More information available here: