Zimbra includes out-of-the-box support for 2-factor authentication, email encryption, secure communications via TLS and HTTPs, and more. Zimbra also integrates easily with third-party security apps.
Two-Factor Authentication: Zimbra includes two-factor authentication, which increases the security of your mailbox by adding a physical layer for login.
Zimbra MTA Postscreen: Zimbra Postscreen keeps spambots at bay by making more SMTP server processes available to legitimate clients and delaying the onset of server overload conditions.
S/MIME Digital Signature & Encryption: Zimbra Collaboration Network Edition offers email cryptographic security services such as authentication, message integrity, non-repudiation of origin (via digital signatures), and privacy and data security (using encryption).