This indicates that the connection is not reaching Zoho, and the local server is responding instead.
There is a setting in WHM called "SMTP restrictions" which could have been enabled preventing emails being sent through other mail servers. Please double check this setting to make sure this is not enabled. This can be found in WHM at Home > Security Center > SMTP Restrictions
https://support.cpanel.net/hc/en-us/articles/1500009931961-How-to-disable-SMTP-Restrictions
