I spent the last week trying to break Bongo. Two small Hetzner VMs, 2 shared vCPUs and 4 GB each, one running the server and one throwing mail, IMAP, CalDAV and POP3 at it. The point was to find what breaks first. It held 10,000 accounts with about 820 connected clients for 15 hours without losing a message. The limit was CPU: search indexing and bcrypt at login. 15,000 idle IMAP connections sat at 9.5% CPU. The load test found seven bugs. All seven are fixed and released in v0.15.13. I would much rather find these on a test VM than on someone's mail server. These numbers are from one run so far. Details: https://bongo-project.org/load-test/