If you as an individual want to write an email to your family, what would you do? I would open Gmail and compose a letter to my family. Maybe I would add a picture as an attachment.
If your brand wants to send an email to its customers, what happens? A graphic email is designed, content is forced in to it, it's cut & coded in non-semantic HTML, and then distributed to a mail list. Upon arrival, the images don't load because they're probably disabled by default, and your table layout breaks because email clients don't support web standards.
Email clients don't support web standards because they're not web browsers. Microsoft goes so far as using Word as Outlook's rendering engine. That makes sense: email is correspondence, not interactive media. That's why we prefer plain text emails for our newsletter. It looks more personal, and has a 4-7% higher click-through rate.
Stop trying to shoe-horn your branding design guidelines in to your emails, and start communicating with your customers instead.