It isn't standard for APIs to perform this kind of check -- not that they couldn't, but it isn't a defect if they don't IMO. Field validation at the UI level is usually best-effort (especially for things like emails) and it's actually a good thing that you have a way around it. For one example, if your CRM uses non-traditional ways of storing email addresses, such as the old Lotus Notes messaging format, you don't want to lose that data or block the sync, or if your CRM allows a user to store an address with angle brackets, and so on.
Since Sugar is letting you store multiple email addresses, don't you have to create a workaround in Sugar? Seeing 'Array' suggests that toString() was run on a PHP array that doesn't have an overloaded serializer. That didn't happen in Marketo, right?