ckeEmailHTML.config.stylesSet = new string[]
{
"default:/scripts/ckeditor.js"
};
ckeEmailHTML.StylesSet = "[{ name : 'Strong Emphasis 1', element : 'strong' },{ name : 'Emphasis2 ', element : 'em' } ]";
<cke:ckeditorcontrol id="ckeEmailHTML" runat="server" visible="true" width="100%" height="400px" entermode="P" formattags="p;h1;h2;h3;h4;h5;h6;pre" stylesset="default:/scripts/ckeditor.js" />
<cke:ckeditorcontrol id="ckeEmailHTML" runat="server" visible="true" width="100%"
height="400px" entermode="P" formattags="p;h1;h2;h3;h4;h5;h6;pre"
stylesset="[{ name : 'Strong Emphasis 1', element : 'strong' },{ name : 'Emphasis2 ', element : 'em' } ]" />http://svn.ckeditor.com/CKEditor/trunk/ ... default.js

Re: ASP.NET and StylesSet
Are you sure you have the the newest version of CKEditor for ASP? If I copy your example:
ckeEmailHTML.StylesSet = "[{ name : 'Strong Emphasis 1', element : 'strong' },{ name : 'Emphasis2 ', element : 'em' } ]";or<cke:ckeditorcontrol id="ckeEmailHTML" runat="server" visible="true" width="100%" height="400px" entermode="P" formattags="p;h1;h2;h3;h4;h5;h6;pre" stylesset="[{ name : 'Strong Emphasis 1', element : 'strong' },{ name : 'Emphasis2 ', element : 'em' } ]" />It's work fine.If you want use default, please try in this way:
or