If your senders select a Sales Insight email template, the Email Template upon which it's based controls the font and will override that default setting. Below is the HTML in the <head> section we use to select the Calibri font that is the default in our Outlook emails.
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
<STYLE type=text/css>
P {FONT-FAMILY: Calibri; FONT-SIZE: 11pt}
LI {FONT-FAMILY: Calibri; FONT-SIZE: 11pt}
</STYLE>
</head>