You might be spoofed. To identify and resolve this do the following:
Follow these steps to identify and resolve this issue.
1.Check your email’s header to see if the spam is coming from your Media Temple (mt) server or not.
2.Check the IP address or the name of the server the spam message was sent from. If your server’s name is there or gridserver.com, then you may be sending the spam, and it could indicate that your email is hacked. But if the original server is unknown, it's likely that you are spoofed which means that somebody is pretending to send those spam emails using your email address. However the emails are not coming from your server.
3.To overcome spoofing you should create SPF record of your domain.
To create an SPF record for your domain check this