The easiest way to do this is with Word Mail Merge. When you use the mail merge wizard, the first thing you will select is the document type. One of your choices is Email Message. Later, you will get to select Outlook as your source for the email addresses. Even nicer is the fact that you can include values from your contacts in your email text by including fields.
Each email is sent separately with its own To: address.
There’s a tool called ReliefJet Essentials which does this. The feature is called Send Messages Personally and can be found at http://www.reliefjet.com/Product/Features#Folders. Works perfectly.
In case anyone is still looking into this and isn’t wanting the more advanced mail merge option:
Complete the first email without adding the ‘To’ address, click in the ‘To’ box and hit Ctrl+F. That should bring up an exact copy of the draft you just created. That saves you from having to copy and paste the subject, body, and attachments.