Add Email to Salesforce allows you to push any emails you receive in Outlook to your Salesforce account. We do the look up based on the sender of the email, so we can currently only use this button for emails you received, not emails you sent.

We're working on changing the functionality to allow you to log emails you've already sent. In the meantime, if an email didn't automatically log when sent, or show up in your failed activities in the activity log, you can forward it to your Salesforce BCC email address.