Inline CSS is styling that is added directly to HTML markup using the
style attribute. If you're linking to or importing stylesheets within your email code, it's a good idea to inline your CSS. Otherwise, your styles may not be correctly applied.
CSS inlining tools, such as MailerMailer's Magic CSS Inliner, will help you move your styles inline. Within the tool, simply enter your email's HTML or a URL to the email if it's hosted online. Then, press 'CONVERT IT' and your inlined code will be available for you to copy and paste into your HTML campaign.
If you're fetching a campaign, you will have the option to inline all CSS within step 2 of the campaign creation process. In step 2, simply select the checkbox that says 'Inline any css found'.