Email clients typically support only a subset of valid HTML, and don’t have strong support for CSS (especially CSS in the
<head> of the HTML email). Yahoo, Outlook, and even Gmail strip CSS that’s included in the
<head> of your HTML, so in most cases CSS for email is handled using inline styles.
Rather than inlining your CSS manually before sending your HTML to Mandrill or when you’re generating a template, we offer a sending option to inline CSS automatically at the time of send (as long as your message is 256KB or smaller). Set this on the Sending Defaults page in your Mandrill account, on a per-message basis using the
inline_css option (API) or
X-MC-InlineCSS header (SMTP), or using the Mandrill Rules Engine.
CSS inlining can be an intensive process and because Mandrill is designed to get your emails out as quickly as possible, the extra processing time to inline CSS can affect delivery speeds. If you have a larger template or HTML document, it's best to inline your styles manually or by using a CSS inlining tool as a part of your sending process.
Mandrill will inline CSS only for HTML emails smaller than 256KB.