It has been a while since I used Gnome or Evolution mail; I used to have a pair of plugins, evolution-on and evolution-rss, I could find the Rss, but I cannot find evolution-on; not on the fusion repos.
So, if anyone is using it, may I ask how you got it?
I think the problem is that the GNOME developers consider system tray to be obsolete.
The closest replacement should be GNOME Shell Extensions, e.g. New Mail Indicator.