This is NOT thread on what you use, but on what is the most powerful email client. I personally use Outlook 2016, Apple Mail, and Thunderbird. I find strengths and weaknesses in all of them. Features that Thunderbird has, are missing from mail, and features Outlook has are missing from Thunderbird. For the most powerful email client award I may give it to Thunderbird. Although it's lacking some features, has limitations, has bugs and such it seems to have the most features. Unfortunately I do not believe its in development any longer.