Certificate Error

[patrickwilson82]

Several of my users are getting this message "A connection required to synchronize the Global Address list could not be secured due to a problem with the server's certificate. Click view certificate for details.

You may authorize this connection, but your mailbox will be vulnerable to intruders."

My self-signed certificate is supposed to be good until at least 2022. But this error is also not happening on all machines. When I contacted Zimbra Support, they told me that I need a commercial certificate, and that's why this is happening. Can anyone confirm that, or elaborate a little further? Thanks.