I’ve had a nearly perfect experience using Postbox (https://www.postbox-inc.com/). The interface is slick, the license is per-user rather than per-machine, and it doesn’t bitch at me when I’ve deleted an email and immediately close the application. And it turns out that their support is excellent…
An employee has started using Microsoft Outlook 2016 for the Mac. At our company we haven’t been happy with the capabilities of using any other product for calendar invitations. It turns out, though, that shiny new Postbox coughs giant hairballs when trying to download and read invitation emails from Outlook 2016. It just simply fails to display the email or mark it as read. Postbox support gets kudos for their efforts on diagnosing the problem, but acknowledge I’m experiencing something wack. They’ve offered a refund.
I’m now using Airmail 3.