Zimbra Version: Release 8.6.0_GA_1153.RHEL7_64_20141215151110 RHEL7_64 FOSS edition
Based on Zimbra Article KB 22434 : https://wiki.zimbra.com/wiki/Installing_a_LetsEncrypt_SSL_Certificate
Log on Zimbra user then stop proxy and mail box service for renew proccess.
su zimbra