The Zimlet gallery is the best place to find and publish extensions for Zimbra.
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Zeta Alliance - Zimlets
Zeta Alliance - Maldua Zimbra OSE 2FA Extension
GitHub - maldua-suite/zimbra-ose-2fa: Two factor authentication for Zimbra OSE
Zeta Alliance - Users
How to enable dual authentication factor in Zimbra (2FA) – bTactic Open Source&Cloud Solutions
Zeta Alliance - Zimlets
Zeta Alliance - Users
Two-Factor Authentication for Zimbra Open Source Edition – Ahmad Imanudin
Zeta Alliance - Users