In this article Mathew Patterson discusses the technical, design and information elements that make up a successful HTML email.

Here are the quick and dirty guidelines:

• Don’t waste your readers’ time — An email inbox is a busy place, you won’t get much attention.
• Permission matters — Not only do you need to have
  permission to email people, but it helps to remind them of how they
  gave you permission, as specifically as you can.
• Relevance trumps permission — Just having permission is not enough, the content you are sending must also be relevant.
• Make unsubscribing easy — There’s no point emailing people who are not interested.
• Image blocking is common — You can’t rely on people actually seeing your images.
• Bring back tables — Structural tables are still often necessary for creating columns.
• Add inline styles — Gmail removes anything else.
• Don’t forget your plain text version —  You can make blocks of text more readable.
• Meet your legal obligations — For example, CAN-SPAM for US senders.
• Test, test, test — It’s the only way to be confident about your design working.

Read the full article on the Campaign Monitor blog.