I wish to share some ideas in order to improve the mail client, which is the most used by me. I used Thunderbird for years: I feel it’s an outdated app, poor maintained, but some features are very useful.
=== Rich editing
I really miss the quick font picker.
Very often I paste snippets of code to a mail, and I need to format it with a monospaced-font. Any will do. The font picker is nice, but -at least- offer two-three latest selections on the drop. Persistent, of course.
Same consideration for the text color.
I really miss a better table editing.
The Thunderbird table editing surely isn’t a masterpiece, but you can add/remove rows/columns with ease, visually.
The em-Client table editing is rather primitive, counterintuitive, and sometimes unstable (especially when copy/paste cells content). Being so, I’d prefer to type HTML by my own.
=== Contact color/icon
I’d like to assign a color/icon/avatar to each contact so that it will be easier to find it in the message list. Now all is plain text, just black and white. I must “read” the name for every single message, and that’s sometimes confusing.
=== Quick filter toolbar
Consider a configurable toolbar/list, whose buttons (or checkboxes) feature the color/icon as above described. By simply pressing a button (or checking it) the message list will show only the selected contact. More contacts selected, the list will show the union of the involved messages (OR-like).
I hope to have given you some useful hint.