Mailnag is a Popper fork for GNOME Shell. It description on Launchpad says it all: "What Popper is to Ubuntu’s Unity (messaging menu), Mailnag is to gnome-shell.".
So what’s Popper? Well, it’s an application that uses POP3 and IMAP servers and notifies you about new emails you receive (the number of emails, subject, sender, etc.).
Mailnag is currently in early development stages but the basic email notification is working fine – tested by both the developer and me under Fedora 15.
Here are a couple more screenshots:
To use Mailnag, you must get the source code via Launchpad but you don’t have to compile it since it’s written in Python. So to use it, install bzr and then get the code using the following command:
bzr branch lp:mailnag
Then run the "mailnag_config" file and enter your account details, make any other changes you want and close it. After this, you don’t need to run anything else, Mailnag should automatically start and you’ll get a notification as soon as you get a new email (well, Mailnag checks for new emails every 5 minutes by default, but you get the idea).