You can target Yahoo! Mail by using the @media yahoo media query and adding your styles.
HTML Email Knowledge Base
Using CSS media queries to target Apple iPad in both portrait and landscape view.
The Outlook email clients add a spacing above and below images which have a height smaller than 19 pixels.
Gmail and Yahoo! Mail add a spacing between images. This issue first showed up when viewing emails in Firefox and later spread across other modern browsers including IE, Chrome, Opera, and Safari.
Some email clients and webmail services such as AOL Mail, Gmail, Lotus, Outlook, Thunderbird, and Windows Mail will display a blue or black border around images that are linked but don’t have a border specified.