[tomjen3]

Thunderbird is free, Outlook comes with an office subscription, most people probably just use Gmail.

Then there is Apple Mail if you are on Apple.

These are all good clients. I have never heard of Spark, is it that much better?

Edit: looking through their feature set, the answer is no. It doesn't seem to have a feature Thunderbird doesn't.

[agiacalone]

I used Spark for a while, as it supported Exchange/O365 out of the box which was one advantage over Thunderbird. It also has decent mobile support.

But Thuderbird is a superior email client, IMHO