Simplifying, Textpack is a markdown file, plus optional image files, packed into single zip file.
See attached video to see how to configure email account in Keepmark.
In Keepmark you can configure other external tools to work with. Mainly, I can be other applications, or shell scripts run for the selected document. Before you start you should configure them in Application Settings.
In Keepmark you can add relations for an existing document, to other documents, tasks, or people. This can be done through the side panel of an open document.
In Keepmark you can configure default author for new notes. It can be helpful, when you want to discover if note is created by you, or copied from external source. This operation you can do in Application Settings.
Keepmark has support for spell checking when creating notes. To enable this functionality, you must first go to the app's settings and download the dictionaries of your choice.