I have been using Mailbird as my email client on my work PC since it was available about a year ago. Although I religiously keep my inbox empty by practicing Merlin Mann’s Inbox Zero strategy, I only today discovered an Easter egg hidden on the Inbox Zero screen. Hovering your mouse over the o in the word Zero causes the o to darken. Once it has darkened, click on the o.
Wait a moment and you will hear a midi-style jingle and see a screen titled “Mail Invaders.”
Then you will be able to play mail themed Space Invaders game.
Thus far I have found no documentation of this Easter egg, but I am sure it will be pointed out to me if I am wrong. Enjoy!