You can send plain text emails, HTML emails (designed like a web page) or even emails enhanced with rich media such as Flash, a programming language to create animations.
Generally, when a person subscribes to an email newsletter, you should give him the choice between HTML or plain text. Most readers prefer HTML, but with graphics being blocked by some spam filters, users’ preference tends to lean towards plain text emails these days.
Several email software programs can detect if the reader’s email program can read HTML (That’s called “sniffing”) and will automatically deliver the right version. But be aware that you should always provide a plain text version of your HTML emails for readers who prefer or need them.
To be absolutely sure your message gets delivered, you can choose to add a top link to your HTML email, referring to its online version.
Conclusion: Always create a plain text version of HTML emails for users who prefer or need them. Using a tool that supports sniffing techniques will help you in getting the job done properly.