HTML email - include external CSS styles or use inline CSS

I need to create and send an emails with HTML content in PHP. Should I include external CSS styles into the email source code or is it better to use inline CSS styles ?

Hi,

It is not recommended to use external css file to apply the styles on email with HTML content. Email clients could block external links, so the email content will probably not appear as you like.

Using of inline CSS is more complicated method how to style HTML email, but I would say it also more reliable method to appear email content as it should.