OUtlook mail, and any other mail utility, is not a browser.
They do not necessarily render all HTML, and more so, CSS.
That being said, you also need to check browser settings to make sure the email was marked as HTML vs RTF vs plaintext.
All that being said, setting the style of the hr element to border=0 does not actually guarantee that it will make it go away. It has default properties.
] You really should just delete
it if you don't want it - which is the most sensible way, is it not?