Normally when declaring fonts I would use something along these lines:
font-family: 'Gill Sans', GillSans, Arial, Geneva, sans-serif;
Which, according to web standards is correct. If sending emails to email clients this also works except Outlook 2007 (and possibly 8). A possible solution was to declare the font-family twice:
font-family: font-family: 'Gill Sans', GillSans, Arial, Geneva, sans-serif;
But this doesn't work for me. It seems there is a much simpler solution:
font-family: 'Gill Sans', 'GillSans', 'Arial', 'Geneva', 'sans-serif';
Adding single quotes around all font does the trick!