The main one, because it's there and it works perfectly fine for my use case
Outlook.
Nothing else comes close in terms of features.
That, and it's regularly maintained.
i second this! It is pretty intuitive, has nice gesture implementation.
I’m sure there are email apps with a little more design beauty, but outlook reigns.
I use Spark on iOS and Mac, and I love all the extra features (smart inbox and follow up reminders mainly). The only reason I don't use the Gmail anymore is because I have multiple addresses now.