Assuming that it concerns the composition of an outgoing mail (not hyperlinks in received mail), my understanding is like this:
- If you compose an unformatted (plain text) email (uncheck "rich text"), the hyperlink will be automatically highlighted, not immediately in the composed message, but in the copy-to-self.
- If you compose a formated (html) email, ("rich text" checked), the behaviour above will depend on whether the hyperlink is formatted as hyperlink, which you can control using the respective button in the format bar in the composing message editor.
However, having said that, the behaviour on the recipient's side depends on how the recipient's client handles such hyperlink. Only in case that you have specifically formatted it as hyperlink in an html message, you can be sure that it is always highlighted and clickable regardless whether the recipient opens the message in a client, over a website or whatever.