We used to work with a small MAP tool called Click Dimensions, and we had this feeture of test a single email in differente email provders. So our designers team were able to see how a email would look in all versions of Outlook, Gmail, Hotmail, Yahoo, AOL and so on.
Do we have something similar in Marketo, so we dont have to send tests to a different email addreses and hope that it will look "good" in those that we cannot test?
You can use litmus or emailonacid. It is not integrated directly. Easy to post the code or send samples to do the same thing.