There are 2 types of message types available when sending an e-mail;
Rich Formatting
This type will allow you to add styling to text, and provides additional features such as tables, lists and text justification. When using this message type the message will be sent as a multipart message (HTML and Plain Text), so where HTML is not available on the recipient's e-mail client the Plain Text version will be displayed.
Plain Text
This type does not provide any styling or layout facilities provided by Rich Formatting. The message will be sent as text only.
When replying to an e-mail message the default message type will be set depending on the type of the original message (i.e. If the original message was HTML formatted then by default the message composer will provide Rich Formatting).
You can switch between Rich Formatting and Plain Text message composition by clicking the button above the message composer. If you have already started writing your message it will be converted (as closely as possible) to the alternate format.
The default message composition type (for new messages) can be set in User Preferences under the Settings menu.
