I had some troubles with Outlook falling back to Times New Roman when using a custom font with @font-face declaration. Not only did I have to hide the @font-face declaration from Outlook using the conditional around it's own style block. (all other styles go in another block). I also had to double wrap my textual content in spans with the conditional tag. Just to give an example of how this technique as posted by @CodeMoose (above) works while using a custom font.
<!--[if !mso]><!-->
<style type="text/css">
@font-face {
font-family: 'Museo100';
src: url('http://www.somesite.nl/site/fonts/museo100-regular-webfont.eot');
src: url('http://www.somesite.nl/site/fonts/museo100-regular-webfont.eot?#iefix') format('embedded-opentype'),
url('http://www.somesite.nl/site/fonts/museo100-regular-webfont.woff') format('woff'),
url('http://www.somesite.nl/site/fonts/museo100-regular-webfont.ttf') format('truetype'),
url('http://www.somesite.nl/site/fonts/museo100-regular-webfont.svg#museo100') format('svg');
font-weight: normal;
font-style: normal;
}
<!--<![endif]-->
First I tried to put the conditional around my "Museo300" font declaration inside the inline style but that obviously didn't work, so I had to double wrap my content into two span's with style declarations. The inner one being conditional for non MSO.
<span style="color: #00B2EB; font-family: arial, sans-serif; font-size: 14px; line-height: 19px; font-weight: normal;">
<!--[if !mso]><!--><span style="font-family: Museo100;"><!--<![endif]-->
Text goes here, shown in Museo in Apple mail while this method shows in Arial in Outlook (and others that do not support custom fonts
<!--[if !mso]><!--></span><!--<![endif]-->
</span>
This works great in getting Outlook to show the text in Arial while Apple mail will show the text in font Museo. Other clients (like mail on Android) have a normal fallback behaviour and just show Arial.