Mailnag is an email notification daemon for POP3 and IMAP servers. It comes with predefined settings for Gmail, GMX, Web.de, and Yahoo email services, and it offers features ranging from triggering popup or sound notifications on new email arrival, a spam filter, or running an user defined script when receiving new emails.
Over the years, Mailnag has supported a wide range of desktop environments: Gnome Shell, Unity, Pantheon, Xfce and Cinnamon. Nowadays though, only the Gnome Shell extension is actively maintained (and the only plugin in the Debian/Ubuntu repositories).
The Gnome Shell Mailnag extension (needs Mailnag daemon to be installed on the system) shows an unread email indicator in the top panel. The indicator menu shows new email details like the sender, email title, date, and a small email body excerpt, as well as options to mark all emails as read, and to check for new emails.
With the new Mailnag 1.3.0 release, the settings window got a modernized user interface, with headerbars:
What's more, the new Mailnag version includes support for mbox and maildir backends.