This is the safest format for email, 'what you see is what you get'. The text is plain ol' 'Times New Roman' which cannot be changed, you can't add colour or change the test size. It's like writing in a basic text editor, like Notepad.

The emails remain small, and is compatible with all email applications.

This allows basic formatting of text, enabling changes to the font, text size and adding a bit of colour to the text. Using this format is probably ideal for people who like to be expressive with their emails without the 'bells and whistles' of images. The nearest comparison is writing in WordPad.

RTF is not compatible with some email applications, which will convert the email back into plain text without any difficulty. Internet mail doesn't support RTF, and will convert it back to plain text.

This format has the ability of imatating web pages, and therefore can look fairly impressive. Images can be made part of the email itself, scripting in the background can be used to perform various functions. However because of this it makes the user vulnerable to viruses and unorthadox advertising tactics. If code can be activated purely by displaying the email content in a preview pane, then the ability for malicious code to be run becomes very easy. Viruses spread like wildfire this way.

Advertisment emails, (spam) can use HTML emails to monitor the number of people who have read, or at least opened their email. By embedding code in the email (perhaps unique to that email) or a link to an image on their server as part of the email, they can monitor their success.

HTML emails tend to be fairly big, as they contain basically a web page. The more graphical content, the larger the email will be.

HTML emails are the least compatible out of all of them, different applications and internet mail sites all utilise the format slightly differently, this can give some applications problems in intepreting the content correctly. If the application cannot interpret the email, it will either display the entire HTML source code (what a mess!), or it will just display a blank page (perhaps deleting the message). This is not common, however does happen.

Some companies automatically convert HTML email to plain text, stripping all the formatting, and changing any graphic's into attachments. This is mainly done as a security precaution, to help fight the threat of viruses.