Outlook offers three email format types for creating email messages: plain text, rich text and HTML. By using the HTML email format, you can insert HTML tags, such as images, links, forms or scripts, into your outgoing email messages. Be aware that if you send email messages as HTML format only you are making the assumption that the receiver can view HTML formatted mail, which isn't always the case. To avoid viruses and malicious code most mail packages will not open email that includes HTML that calls external web addresses or images, or contains scripts or other executable programs, and these should be avoided.
You will need to turn on HTML editing for emails. Click Tools > Mail Format and from the drop down menu select HTML (when creating new mails you will also have the option to change the message format). You can now put HTML code within your email.