To my surprise, zimbra is creating a lot of redo log that will clog up your server and eat disk space. After researching on the net today, I found out that the redo log could be deleted safely without affecting your zimbra server. But the services must be disabled first and for safety, only delete redolog that at least a month old.
Stop the service as Zimbra user
su – zimbra
Go to redo log archive directory
List the files
Just delete the files that at least a month old. You could use something like rm -f redo-201806* to delete all files that start with redo-201806 in its file name.