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We are running a Zimbra 8.6.0 mail server on an Ubuntu box. In mail log I am seeing some Anonymous TLS connection made to our server mail server.Is this something I should be worried.

Jun 11 12:14:15 mail postfix/smtpd[2963]: Anonymous TLS connection established from p3plsmtpout004.prod.phx3.secureserver.net[208.109.80.1]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)

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No, this should not worry you at all. "Anonymous" simply means that the connection does not use an authority-issued certificate (that people usually buy), and that the TLS connection in itself therefore does not guarantee that the mail is coming from p3plsmtpout004.prod.phx3.secureserver.net. What is guaranteed is only confidentiality against passive eavesdropping, not against impersonation and man-in-the-middle attacks.

The "anonymous" part therefore means that the security is less than for a non-anonymous connection, but it's still much better than not using a TLS connection at all.

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