What is a good project to learn about distributed systems and networking?

Since one of the best ways to learn about something is by doing, I would like to learn more about distributed systems with perhaps some networking topics sprinkled in. I’ve thought about implementing my own email server as an option, or perhaps creating my own p2p cluster using some Raspberry Pis. Would be curious to hear from the community what your thoughts are about this and if you’ve found a particular course/book helpful.

Note: I’m completing the excellent nand2tetris course which has been a phenomenal experience. One of the best courses I’ve taken.