display:none doesn't work on Outlook 2007
Outlook 2007 uses the Microsoft Word engine for rendering HTML which has very limited support for CSS. This page describes the kind of things you can expect to work (display is one of the "not supported" ones).
Unfortunately, there's not much you can do. You can enclose the element in HTML comments <!-- ... --> which would stop it from rendering, but that's about it.
list-style-type: none does not work in Outlook 2007
As you can read here: http://msdn.microsoft.com/en-us/library/aa338201.aspx "none" isn't working. But you could fake a list with a table. Or, as you don't want any list-type, just do line-breaks.
Gmail is ignoring display:none
If style="display:none" does not work in Gmail, put style="display:none !important;"
and it works in Gmail.
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