Lost access of the admin user

Lost access of the admin user

Hello,

We no longer have access to Zoho control panel where we can manage the emails. We don't know the phone nr nor e-mail used for it. 

The client is currently using subaccount rb@rentacarmalta.eu

It seems something happened with the SMTP which we are using in our website and I suspect it is because the free accounts are no longer supporting the SMTP and we need to upgrade to paid account. 

Please help. What are the steps to recover our admin account?

Thank you