With Outlook, you can send messages in three different formats:
Plain Text
Plain Text sends emails using just plain text characters. All email applications support plain text. This format is great if you do not depend on any fancy formatting, and it ensures maximum compatibility. Everybody who has an email account will be able to read your message. Plain text doesn't support bold, italic, colored fonts, or other text formatting. It also doesn't support pictures that are displayed directly in the message body, although you can include the pictures as attachments.You should note that Hubspot has found that Plain Text messages get a higher open and click rate than HTML messages.
HTML
HTML lets you use HTML formatting. This is the default message format in Outlook. It is also the best format to use when you want to create messages that are similar to traditional documents, with various fonts, colors, and bullet lists. You can make text stand out with italics, for example, or change the font. You can even include pictures that will display inline and use other formatting tools to make your messages prettier and easier to read. Today, most people with email can receive HTML-formatted messages fine (although some prefer plain text for purity's sake). By default, when you select either of the options that allow formatting (HTML or Rich Text), the message is sent in HTML format. So when you use HTML, you know that what you send is what the recipient will see.
Rich Text Format (RTF)
Rich Text is Outlook's proprietary message format. RTF supports text formatting, including bullets, alignment, and linked objects. Outlook automatically converts RTF formatted messages to HTML by default when you send them to an Internet recipient so that the message formatting is maintained and attachments are received. Outlook also automatically formats meeting and task requests and messages with voting buttons so that these items can be sent intact across the Internet to other Outlook users, regardless of the default format of the message. If the Internet-bound message is a task or meeting request, you must use RTF. Outlook automatically converts it to Internet Calendar format, a common format for Internet calendar items, so that other e-mail applications can support it. You can use RTF when sending messages within an organization that uses Microsoft Exchange; however, we recommend that you use the HTML format. This is a Microsoft format that only the following e-mail applications support: Microsoft Exchange Client versions 4.0 and 5.0; Microsoft Office Outlook 2007; Microsoft Office Outlook 2003; Microsoft Outlook 97, 98, 2000, and 2002
