What does it do?

This will force Outlook 2010 and 2013 to render the height of an element to your desired height. This is only needed if your height needs to be set less than 15px. You will also need to set the font-size and line-height to the height you need. This can be used to create small amount of space between elements or divider lines. The mso-line-height-rule:exactly can be set on a td or table, or it can be set in the head styles.