I have been looking through your user documentation and cannot find any details of the level of support provided for users who rely on accessible technology or accessibility features provided by the operating system.
There are three aspects of this question:
a) Support for screen-readers, such as VoiceOver (on Macs), Microsoft Narrator, NVDA, JAWS
b) Support for high-contrast schemes. Windows and OSX provide features to adjust the styling and colours. With windows High-contrast dark scheme enabled, the emClient still uses a white background for its display. Ideally, emClient would use the OS definition for colours and fonts, or alternatively you could have a styling configuration file which can be modified to take account of different eye conditions.
c) Support for changes in screen presentation layout, to handle conditions such as Homonymous hemianopsia, where it would be ideal to put the list of mail folders on the right hand side of the screen.