Sometimes you will get a bounce-back message or notification from someone saying you sent an email that you didn't. What can you do about it?
There are two possible reasons if someone gets an email from your account that you have not sent:
Your email address was spoofed. Spoofing is when an email is sent from one address but it appears to come from another. There is nothing you can do about this. Usually this means someone that has your email address in their contact list is sending out spam. The spammer uses spoofing to hide the true sender, thereby extending the amount of time they can send out spam before they are found out and stopped. Usually the spammer will rotate through the contact list so your good name should only be sullied for a short time.
Your email account has been compromised. This is more serious and will require immediate action on your part. The first thing you need to do is to log into your email account and change your password. If you cannot log in then it means your account password was changed and you need to follow the procedure to reset this as soon as possible.
How can you know which case is true?
Log into your account and look for signs like new rules, your password was changed, emails in your sent items that you did not send, enormous amount of bounce-back emails. All these can mean you have a compromised account. Change your password immediately.
If your account has been compromised and you eventually can get everything under control, there are a few things to check in your account.
First, look to make sure there is no rule setup to delete all or some incoming email. This is another trick the spammer uses to hide the infection. The rule deletes the bounce-back emails and angry or questioning replies from your contacts so you have no idea your account is sending out emails.
Second, verify your security info, including your recovery email address and contact info. Sometimes the spammer will change this info so they can reset your password at a later date to use your account to send out more spam.
Finally, know that you are not the first, and won't be the last to fall for their tricks. Embarrassing? Yes, but it is not the end of the world, and no one should throw stones because it might happen to them one day.
The following article goes into greater detail and can be useful as well