My ZImbra 8.6 was throwing the following error once after my letsencrypt SSL got expired:

Unable to start TLS: SSL connect attempt failed error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed when connecting to ldap master.Cannot determine services - exiting

The Fix:

In order to fix the problem you need to modify the following parameters as Zimbra user:

zmlocalconfig -e ldap_starttls_required=false
zmlocalconfig -e ldap_starttls_supported=0

Then restart zimbra by using

zmcontrol restart

Hope it’ll restarted