1.stalwartlabs / stalwart
All-in-one Mail & Collaboration server. Secure, scalable and fluent in every protocol (IMAP, JMAP, SMTP, CalDAV, CardDAV, WebDAV).
+720 stars
from 14,905 · score 6.58
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2.flectar / mail
Built from the ground up for speed. Flectar Mail delivers native performance, instant startup, and as little as 80 MB of RAM.
+536 stars
from 1,189 · score 6.29
3.cypht-org / cypht
Cypht: Lightweight Open Source webmail aggregator [PHP, JS]. Supports IMAP/SMTP, JMAP and EWS (Exchange Web Services)
+49 stars
from 1,738 · score 3.91