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Here is another reason why I cannot replace my desktop email client:
1. All good desktop email clients have keyboard shortcut keys.
2. Make sure all flagging actions have shortcut keys (Mark as Read/Unread - CTRL-K, CTRL-SHIFT-K), Folloup/NoFollowup, Spam/Not-Spam - CTRL-J, CTRL-SHIFT-J). Careful at browser default shortcuts, those should not be used.
3. Shortcuts for basic actions (Forward, Reply to All)
4. Nice feature: assign a shortcut key to each folder, so you can move mails to that folder just by clicking the shortcut (for example CTRL-1..9)