One of the advantages of my MacBook Air Mountain Goat migration is that I am being exposed to new applications.
Whenever I need an app, I just grab whatever Ubuntu offers as the default before I go looking for an alternative, if needed.
Of course Thunderbird has been around for a while, but I hadn't tried it out in several years, and had pretty much become a slave to Apple's "Mail" program.
Most impressive is how it archives all my email and folders from the server and stores it in a local database that has blazingly fast contextual search. Beautiful.
T-Bird's spam function is also outstanding. It outperforms all programs I've used before and even lets you train it's filters before you enabled the filing/deleting of spam messages.
The only weird thing I've noticed is that the default setup places your repliy to an email at the bottom of the original, which does not seem standard email practice. It is easily changed in the Account Prefrences pane.