If it appears that someone is sending spam from your email address, there are two possibilities. One is that your email account password has become compromised and used to send spam. Typically when this happens you'll receive notices from the server in the form of bounce messages indicating that you have reached your maximum number of emails per day, or that you've reach your maximum deferred or bounce message per hour. In this case, you should follow the instructions to change your email password.
Alternatively, the problem may be that your email address is being forged. In this case, you will typically receive many bounce messages to an email account for spam messages you never sent. The best course of action is to adjust your SPF record to only permit our server to send mail for your domain. To do so, first login the client area, then click on DNS Management under your hosting plan, then click on Manage, under the SPF click on CUSTOMIZE, scroll to the bottom and check the final checkbox, titled "Use the “-all” Entry", and finally click on the INSTALL CUSTOMIZED SPF RECORD button.