It would be great if there was more customization for composing, like default font color, without having to constantly to change it every time to what I want. Also, it would be nice if the popup notifications were able to preview the first line or two of the message.
The default text color can be changed with a template. You will find that in Menu > Tools > Settings > Mail > Templates and Signatures. For a blank template with a different text color, give the template a name, choose a font color and save. Then select that template as the default for new messages. You can have different templates for replies and forwards, or use the same one for all.
For pop-up notifications, this could be there if eM Client could interface with Windows notifications. Still waiting on that!