You can use the license for these purposes:

Free License
Create email templates for yourself and for non-commercial purposes, limited modules.
Business License
Create email templates for your company for commercial purpose, all modules.
Agency License
Create email templates for your clients for commercial purposes, all modules.

It's prohibited to use the license for these purposes:

Partially or completely sell or share the code of the Postcards on any website or torrent.
Create templates based on Postcards code for sale on your website or any marketplace.
Create generators or editors based on the Postcards templates.