That code should be targeting any Outlook clients that came before '15' and Outlook.com but it doesn't seem to work that well.
<!--[if (lte mso 15)|(office365) ]>
<style type="text/css">
.client-fix{
display: none!important;
}
</style>
<![endif]-->
Anyways what I really need to do be able to target Yahoo and AOL but so far I haven't found any reliable information on how to do so.
Please help
As of 2020, with Yahoo having recently updated their codebase, a new way of targetting AOL & Yahoo together has come about, the trick being to have an uncommon unicode element as the ID so that Yahoo will not filter it out: