@titala
Zoho Mail as a service is designed to support Business (one-to-one) communication and we do not recommend sending Bulk mailers using our servers. In addition, we also have restrictions imposed to ensure the reputation of our service, as well as your domain, is not affected.
The limit for the number of emails/recipients allowed in an email when sent through Zoho Mail is high enough to ensure that normal email communication is not treated as spam. They are set as dynamic and are determined by our Spam Engine based on the algorithms we have implemented.
However, we have spam filtering limits imposed due to which you will not be allowed to send any kind of mass emails/ promotional emails or marketing emails. This restriction is common for all users across Zoho Mail irrespective of the subscription you are in. The reason for having these limits is to protect the reputation of our services and also to avoid our IP getting Blacklisted.
Please refer to our Usage Policy on the help page below:
https://www.zoho.com/mail/help/usage-policy.html#alink1
So we would like to inform you that you should have no trouble sending a limited number of emails through your Zoho Mail WordPress plugin.
For further details, please contact our support team: https://www.zoho.com/mail/contact.html