An icon beside the date in the topbar to show if there are unread emails. It checks the notifications in the message tray related to a new mail, no extra configuration needed, very light extension. Do not disable the notifications of your default email client ! It is not a standalone mail checker, you need your email client to be running. Email clients supported at the moment are : Thunderbird (tested), Evolution (tested), Geary (untested). Please ask here to add the client you need. The base idea comes from from https://extensions.gnome.org/extension/1386/notification-counter/ .
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