Universal Email will only take the relative path.
This does not mean that the a relative path cannot be used to go outside of the web root.
say for example the webroot is:
/Volumes/A4/htdocs/
and the page is at
/Volumes/A4/htdocs/mypage.php
the relative path:
../MyRepository
will get you to the MyRepository folder outside of the root.
/Volumes/A4/MyRespository