Writing HTML for email is simple, but that can make it tricky. Confused yet? Let me explain.
Many designers make the mistake of trying to write HTML for email the same way they would for a webpage. Unfortunately, email programs today do not support the same standard of HTML code as web browsers. Code that works perfectly fine in any modern web browser frequently breaks when looked at as an email. Fortunately, writing HTML for email is simple. Anyone who can write HTML with moderate skill should be able to put together a functional email.
Simplicity, as previously mentioned, is the key to writing effective email HTML. Forget what you know about designing a web page, throw out your style sheets and cozy up to using lots tables. The standard for email HTML at this time is non-complicated table layouts with basic inline styles.
Designers should write HTML as if they were designing a web page 8-10 years ago. Things that won't work include, selectors and CSS inheritance. Advanced CSS tricks can be attempted, but cautiously. The more complicated the email becomes, the more chance there is for it to break in front of your customers. It really depends on where your readers will receive your email. There are some situations where risks are worth it and others where it's not. That's a decision for you to make.
Writing HTML for email is as simple as that. It may be counter intuitive for skilled web designers, but it certainly shouldn't be a challenge. If you are an HTML beginner, this is even something you could do. Writing a basic email is probably better than attempting to use an HTML editor or trying to export HTML from Microsoft Word.
Testing is the final stage and it's a very important one. It's how you verify that your exercise in simplicity has produced a quality product. The more testing that you do, the better off you will be while you start to learn to write HTML for email. Don't forget that mobile readers are extremely important these days making up around 45% of reads!