1. Disable ProxySSLUpstream

zmprov ms `zmhostname` zimbraReverseProxySSLToUpstreamEnabled FALSE

2. Reconfigure Nginx

/opt/zimbra/libexec/zmproxyconfig -e -w -o -a 8080:80:8443:443 -x both -H `zmhostname`

3. Configure Zimbra to run both on HTTPS and HTTP

zmtlsctl both
zmprov ms `zmhostname` zimbraReverseProxyMailMode both
zmprov ms `zmhostname` zimbraMailMode both

4. Restart Zimbra service

su zimbra -c "zmcontrol  restart"