Outlook is a challenging client to design for, but our current open tickets in development are for two other issues, I think. One is for background images to work better and the other is related to the Header Title Text that shows up as 1pt text (looks like a dotted line) at the top left of the messages in Outlook.
In the past we have had specific ONTRAmail templates that required tweaking to make Outlook display consistent. I see your ticket with Michelle in Support; I've asked that she obtain the template ID / name for the ticket and escalate it to our QA team so they can test with different Outlook configurations (Exchange server, Office 365, etc.)
I don't recommend using the "Send Test Email" button from within the editor as the final determination, as it only gives a rough approximation of the final display of the email without sending it through the regular processor for populating merge fields, etc. Instead, I use a regular Contact Record instead. (I know Michelle covered this with you but wanted to mention it again here for the benefit of others).