1.stalwartlabs / stalwartAll-in-one Mail & Collaboration server. Secure, scalable and fluent in every protocol (IMAP, JMAP, SMTP, CalDAV, CardDAV, WebDAV).imapjmapmailrustserversmtp+4 moreRustNo license16K total1K forks+1,685 starsfrom 13,940 · score 7.43