As a general rule, if you need to use VML with an email it should actually be defined at the template level (and then if you need to swap out the background image simply use a program-level token) to prevent this sort of thing from happening. Ironically, easier to do non-MSO exceptions when working with Marketo's editor—it respects that syntax a bit better due to it looking more like a traditional comment (i.e.,
<!--[if !mso]><!-- --> Your non-Outlook content<!--<![endif]-->).
Thank you for answer. Yes, it works and make sense, but not convenient way.