Once you link Abacus to your Outlook you will have two icons in the tool bar of Outlook.
Link to Abacus: This allows you to link an email you have sent or received to a contact or matter in Abacus with a few clicks of your mouse. Simply highlight the e-mail, and then click the button. Select the name or matter and click OK.
Link Attachment; this allows you to save the attachment to a separate location and then link it to a name or matter as a linked document.
Now when you send an e-mail you will by default pop up a window asking you if you want to link and send the e-mail or just send and not link. This can be turned off in the user preferences.