The short answer is "Microsoft Outlook".
Not all mail clients support the use of Cascading Style Sheets. Atlassian products use a library called BotoCSS (https://bitbucket.org/mryall/botocss) to inline the styles from .css into the HTML elements when rendering an email to make sure that even broken mail clients will be able to render them as closely as possible to how a web browser would, even the broken old ones like Outlook that don't handle .css correctly.
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