Once in a while, we have received complaints concerning claims of someone else using an email account to send email without the knowledge of the actual account owner.

Cases like this may not really be a hack. But i can be sure to say that your email account has indeed been compromised.

This often creates a problem especially when mass mails are being sent from the accounts. Let's examine this in greater detail and see what can be done.

In the “old days”, that typically meant that your computer had acquired a virus, and that virus was in turn accessing your PC’s email program and systematically sending email to everyone in your contact list or address book. While that’s still a possibility – and you should absolutely make sure that your anti-malware tools are running and up to date – it’s not nearly as common as it once was. Most email programs now carefully protect against unauthorized address book access.

What more likely occurred is that your email account has been compromised – meaning that you probably have an on-line email account, free or otherwise, that someone has gained access too. How this happened is difficult to say. It could be anything from a weak password that’s easy to guess, to your account credentials being sniffed in an open WiFi hotspot, to your simply having shared the account information with someone you should not have and to a friend gaining access to your computer without your knowledge.

Read more at: https://www.todhost.com/knowledgebase/712/How-Did-My-Email-Get-Compromised-and-What-Can-I-Do-to-Stop-It.html