I have the same problem/complaint, but it's not SmarterMail. No email means there is nothing, and nothing is treated as HTTP instead of HTTPS. That popup only appears when you have established an HTTPS connection and the content you are viewing comes from HTTP. If you login via HTTP, you won't get that popup. You don't have to change any browser settings. Just add http://ultimamail.net to your Favorites. Of course the downside is that you aren't using a secure connection.
SmarterMail can't control if people decide to use HTTP or HTTPS sources in their emails (images for example). Not everyone has an SSL Certificate to use HTTPS, and depending on their hosting company, they may have to actually copy their files to a completely different folder to appear on their HTTPS site (this has happened to me before), so SmarterMail couldn't just automatically change all HTTP links/sources to HTTPS because they have no way of knowing if the HTTP content exists on teh HTTPS site.
Sorry, never heard of Maxthon. Not to say that it's not a good browser, but it's certainly not standard (IE, Firefox, Netscape, etc) so I'm guessing SmarterMail never tested their product on that particular browser and Maxthon has problems with whatever SmarterMail is rendering. They can't test every browser that exists and not all browsers are created the same or conform to the same set of standards.
William Eaton, MCSE
iTech Computer Solutions, LLC
www.itechcs.com