I'm not familiar with the codebase, but from my understanding it saves messages in a database [1], then periodically send out a formatted email to people who subscribed to the thread/group [2]

Anyone can post on the forum, you just have to provide an email address (you don't have to register, but they can enforce it)

[1] - https://github.com/CyberShadow/DFeed/blob/master/src/dfeed/w...

[2] - https://github.com/CyberShadow/DFeed/blob/master/src/dfeed/s...