Make sure the com_zimbra_url zimlet is enabled in the COS (or user) and when viewing emails any links will be highlighted, even in text based ones.
If you want to compose a link and display '
some text' instead of
http://www there's a button (world with an arrow) on your HTML compose toolbar.
-You might also manually make it a link (even if you just want to display it as
http://www when emailing a non-Zimbra user, as not all web & thick clients automatically parse http:// & ftp:// etc links as click-able (without further code like <a href= etc) but most do these days.