The most common reason that we have seen for formatting problems is people using Microsoft Word as their email editor or people cutting and pasting from a Word document into an email message.  The message can look "normal" and free of formatting problems, and may even look fine when viewed in the sender's email application, but Word adds hidden HTML into messages which sometimes shows up only after the message has been sent.

Below is an example of a message that we cut and pasted from Word into an email message. It looked normal when we sent it, but came out like this:

Therefore, we recommend that you NEVER use Microsoft Word as your email editor, or cut and paste from Word into an email message. We recommend composing your messages directly in your email application.

If you want to ensure that your message does not have any formatting issues, and can be viewed by all of your subscribers email programs, then the only way to do that is to send it in Plain Text.  Most email programs these days have no problem reading HTML, but Plain Text emails are the only guaranteed method for eliminating formatting issues.