If you’re using Microsoft Office 2010 and haven’t patch paid much heed to e-mails and attachments that you open, you may be open to a zero-day vulnerability which targets Outlook 2010 by using a vulnerability in Word 2010 to gain access to your e-mail and operating system. The attack is delivered through a RTF-formatted Word document or through a RTF-formatted e-mail with hidden elements and code. There’s a temporary fix available which prevents users from opening up affected RTF files, but Microsoft has not announced a deadline for an official fix. This also affects Office for Mac 2011, but not the Office 2013 suite. If you want to avoid these issues for now, open e-mails in plain text only or use alternative software suites to open up your files, like Mozilla Thunderbird or Libre Office.
Source: Microsoft Security TechCenter