I have an issue with HTTPS images in Outlook 2007.
They just seem not to work, and I have not been able to find details about this on the web.
Is it a confirmed limitation?
While I haven't head of this one before, it's probably not the best idea to link to images in a secure directory any way. All that will do is slow down how the images are served, and potentially cause security issues in some email clients. If you send your email with Campaign Monitor, we grab all your images for you when you import your campaign and host them on our dedicated servers automatically. This will get around any of these issues you're having.
Hope that helps, and let us know if you find out any more about the HTTPS issue.
Some clarifications after a few tests (I'm not using Campaign Monitor BTW, the email is for an airline flight booking purchase confirmation).
Basic images called through HTTPS seem fine.
It seems RPC calls through HTTPS are blocked.
For instance, the following call (simplified - will not work as is):
<img src="https://www.myairline.com/en/MyServlet?BARCODE=stuff" alt="barcode">
should display a 2D barcode for us.
It works in all email clients I have tested except in Outlook 2007, where https should be replaced by http.
This is a bit weird to me as the opposite behaviour would appear safer...
Anyway, we've replaced the HTTPS by HTTP already (for all clients obviously).