As of Jan-2023, I'm looking at how to adopt mWeb into a ZettleKasten system. mWeb is an amazing piece of software that supports Markdown. There's also an iOS version so you can keep all of your notes with you.
Consider that you can use pointers in gMail in your zettlekasten. Use this convention to filter all the emails from one sender:
https://mail.google.com/mail/u/0/#search/from%3A(emailaddress%40gmail.com)
A Beginner’s Guide to the Zettelkasten Method - Zenkit
Introduction to the Zettelkasten Method • Zettelkasten Method
Getting Started • Zettelkasten Method
Zettelkasten knowledge and info management • Zettelkasten Method
How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking
Digital Zettelkasten: Principles, Methods, & Examples
Watch - 3M was the company that originally gave employees 20% 'time off' to work on creative endeavours unrelated to their current projects. Google adopted this as well. I believe it is rooted in many disruptive solutions like Postit notes, Glide (from Gortex), and velcro (but I need to confirm!).
Images of Zettelkastens and Luhmann's notes