Lotus Notes and some other email clients do not support the tag in HTML email. Instead, you have to declare the styles directly in the tags. For example:
<table class="Header" cellpadding="0" cellspacing="0"> <tr> <td style="font-family:arial;font-size:14px">Account closure</td> </tr> </table>
For more information, see http://www.sitepoint.com/code-html-email-newsletters-3/.