We are in the process of upgrading our Zimbra mail servers.
The new Zimbra mail server will not support un-encrypted connections.
If you are experiencing issues connection to the new server after the upgrade, see our articles under the Securing Email connections section of our Knowledge Base.
Call our 24/7 Phone Support, they will be happy to help you:
Option #2: Tech. Support,
then Option #1: Email Tech. Support