Hi Florian,
Do you or your recipient use Outlook 2007 at all? Outlook 2007 handles tables very differently from the 2000 and 2003 versions. Outlook 2007 will ignore the font tag within the TD section of the HTML table and revert to a default font.
Outlook contains it's own style sheets which overrides the HTML in the disclaimer, Mail Utilities now contains its own style sheets in version 4.57 to address this knock on affect.
Best Regards
Joshua Clarke
Exclaimer Support