How do I email just one segment instead of my entire list?

Click "segmentation" under the "audience" tab on your dashboard. You will see an "email a segment" button in the top right. 

You can write your post via that button and select the segment to send it to via the dropdown. 

This will ensure your post goes only to that segment.

Please note that emails to a segment are, by nature, not intended for everyone. There is no way to publish a post sent to a segment via web at this time. These are email only.

For further information on segmentation, please see here.