Outlook offers three formats that you can send emails in. Each format has their own set of advantages:
Plain Text - Plain text format does not allow any special formatting within the body of the email message (such as color, fonts, etc.); however, this format can be read by any email recipient and creates the smallest emails.
Rich Text - Rich text format can be read by most email recipients and allows for most text formatting.
HTML - HyperText Markup Language can be read by many email recipients and allows for a variety of text formatting and page formatting.
If you change the default format, all new email messages will use this format (until you change your default again or you change the format for each item).
If you do not see all options choose Plain Text then repeat these steps.