We're currently using Elastic Email to send our email and getting issues with AOL not displaying the message for a few of our users.

The immediate thing we have spotted is that for my AOL account (.com) the message is displayed ok and the page is fully secure (green padlock, no mixed content).
Elastic Email embeds a tracking beacon on a HTTP URL but I think Yahoo/AOL use a proxy so this gets served over HTTPS (e.g. https://ecp.yusercontent.com/mail?url=http://tracking.domain.email/tracking/open?msgid=example-id)

However when the user sent in the bug report the URL for the image doesn't use this proxy (http://tracking.domain.email/tracking/open?msgid=example-id) and this results in mixed content issues which I feel could be why the email isn't displaying.

Testing in Litmus sometimes works and sometimes doesn't, what AOL address TLD is litmus .com or .co.uk (domain our bug reporting user has)?

Has any other user seen these sorts of issues on AOL before?