This article shows a quick and easy way to implement flat user access control system. Flat means, that user access is controlled by level only, which is solution exactly opposite to complex RBAC access systems.
If you are coming over to Yii 2 from Yii 1.x, you may have already read this useful wiki for creating dependent dropdowns.
You can use a similar approach in Yii 2 to do the same. But if you are looking at a prebuilt solution that helps you manage it easier, read along.
These newsletters have always been hard to create as regular web HTML is not properly read by web email interfaces and email clients. I thought that the simplest solution would be to use a web page that would serve as a web version of the newsletter and turn it somehow into newsletter HTML.