Thanks for reaching out! I'm happy to help answer your questions. Safari is definitely a supported browser and should be working as expected. Can you provide details on your Mac OS and Safari version number? On a Mac running 10.12.4, and using Safari on version 10.1, I was able to see the login page as expected.
Also, the abilities to disable / move the preview pane and prevent images from loading in the user interface have been removed in SmarterMail 16.x. Instead, users will find a crisp, clean and secure user interface. Check out some before and after shots to see how 16.x improves the overall look and feel of SmarterMail and why we felt this was a change in the right direction:
SmarterMail 16.x (using Dark Theme)