You can set this as CSS or do it inline so it will force the font choice in Outlook. Outlook is a PITA to get everything correct as each version rolls back their html support.
CSS would be
<style type="text/css">
ul{
font-family: Arial,sans-serif;
font-size:12px;
}
</style>
or inline would be:
<ul style ="
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
line-height: 2em;
font-weight: normal;
font-variant: normal;">
