The Outlook email clients add a spacing above and below images which have a height smaller than 19 pixels.
HTML Email Knowledge Base
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.
By default, Outlook.com centers your email by placing it inside a div with a class named “ExternalClass” using the styles display:inline-block; line-height: 131%. These have no effect when using IE, however in every other browser, the email will not be centered.
By default, Safari on iOS detects any text that is formatted like a phone number and converts this to a link which allows users to call the number by simply touching it.