This is a common problem.
There are a couple of reasons it could happen, but it is not a virus on your MacBook. There are no viruses or other OS X malware known that can cause this to occur on Macs.
The first and most common possibility is that your "from" address is being spoofed by whatever is sending the email. This can occur if you sent an email to almost anyone with a Windows computer running Outlook. Every email address in his address book becomes harvested for spammers to use. This happens all the time, and all you can do is never send email to people who use Windows. This is not a practical solution.
There is nothing you can do about this since it is very easy to accomplish. If you were to post your email address anywhere, even here on this site, there are plenty of automated email address harvesting programs that search all over the Internet for the sole purpose of stealing email addresses for their own nefarious purposes. Make it a habit to never post your email address in a public forum.
A second, more remote possibility is that your email address and password may have been fradulently obtained. If you did not reveal this information on your own, your password may have simply been guessed, or the email server you are using has had its database hacked. The remedy in this situation is to change your email account password.MacBooks iMacs iPods AirPorts, OS X Mountain Lion, 27 years Apple!