I'd recommend against PostBox. It's a Thunderbird derivative, so it has the same security vulnerabilities as Thunderbird does. But it's not updated regularly. The current build of PostBox was last updated in November 2015. It appears to bundle Gecko (Mozilla rendering engine) 7.0.1 which was a part of Firefox/Thunderbird 7 back in 2011. And the only version of Lightning (calendar and tasks) for PostBox is "experimental and unsupported" and was last updated in 2012.

You're better served using Thunderbird and any extensions you'd like to tweak things a bit.