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
2.cypht-org / cyphtCypht: Lightweight Open Source webmail aggregator [PHP, JS]. Supports IMAP/SMTP, JMAP and EWS (Exchange Web Services)phpwebmailwebmail-clientimappop3smtp+6 morePHPLGPL-2.11.8K total256 forks+138 starsfrom 1,649 · score 4.93