Code support in cMail client software
Many people (unknowlingly) use email software that unfortunately, has really bad support for HTML emails.
Lotus Notes based email clients is notorious for being one of the most difficult to please with regards displaying WYSIWYG email content. A number of Linton's clients are major users of Lotus Notes - and the lack of Lotus Notes mail client support for CSS emails is a constant. The old saying "if you can create something that passes through Lotus Notes email clients, you can do anything" rings true with newsletter content.
Basically, creating compliant HTML (or WYSIWYG) emails still requires coders to apply primitive coding and markup using html - and limited at that, to make the best of mostly a worse case scenario. As a developer, you need to ensure that you are not alienating your audience when sending out newsletters - so the old adage of "keep it simple" should be the basis of creating newsletters. We ALWAYS test our newsletters in a variety of email clients to ensure optimum compatibility. Some say that if it passes through Lotus Notes email client, then you can be pretty sure it will pass with just about any other mail client!
Image blocking in email clients: don't "assume" images will work
Many people, either by email client defaults, firewall software, or personal preferences, are blocking images in the HTML-formatted messages they are accepting. Recipients may have to click a button, or right-click on an image to "turn images on." This is also a a privacy protection measure. When you design HTML emails, you should always assume that your images will be turned off by default, and that your recipients may be unaware, or prefer to have them turned off.
You should never have important content contained in images - or else all of your hard campaign work may be totally missed, as in the example below.
Designing for mail client "preview panes"
People think that HTML emails are like web pages. Different email clients have different preview sizes. For example, Lotus Notes preview pane is horizontal, whilst Outlook's preview pane is by default vertical (but can be changed to horizontal).