I hate to say this but InDesign is not a good application for creating HTML for an eblast.
InDesign was created to create layouts for print, PDF, EPUB and DPS. If you have religiously applied character, paragraph, object, table and cell styles, and if you have set up mapping of paragraph styles to HTML tags in the Paragraph Styles panel, you may get something more useful when you export to HTML.
But its a difficult and tricky area. Michael Murphy was done a couple of Lynda.com videos which cover how to improve HTML output from InDesign:
Your best bet for creating email blasts that render properly in most email clients is learning old-school HTML, employing now-otherwise-obsolete methods like inline style declarations and table-based layout. Automatically generated HTML/CSS the likes of which you get from InDesign's HTML export, and even prudent page construction in a seemingly more appropriate application like Dreamweaver, does not work for email blasts. You're in a specialized area that requires an approach that is separate from browser-based page rendering; and InDesign is simply not an applicable tool.