The user is facing an issue with the Zoho Mail free plan, where they are unable to add their custom domain. This limitation is mentioned in the Zoho Mail pricing page, which states that the free plan supports email hosting for a single domain but does not specify details about adding custom domains.

To add a custom domain to the free plan, the user needs to follow these steps:

1. Sign Up: Go to the Zoho Mail homepage and click "Sign-up for free." Choose the free plan and create an account by providing your name, mobile number, password, and contact email address.
   
2. Add Domain: In the Domain Addition page, click "Add now" under the "Add an existing domain" section. Provide your domain address and the name of your organization. Once done, proceed by clicking "Add" and then complete the payment process.
3. Verify Domain: After adding the domain, you will be required to verify it. This involves updating the MX records of your domain to point to Zoho Mail. You can do this by accessing your DNS Manager and setting the MX records accordingly.
4. Configure DNS: Ensure that the MX records for your domain are updated to point to Zoho Mail. This will allow emails to be routed to your custom domain instead of the default @zohomail.com domain.
   

If the user is unable to add their custom domain, they can consider the following alternatives:

1. Redirect from Zoho Mail: The user can set up a redirect from Zoho Mail to their custom domain, which will allow them to use their custom domain for emails. This can be done by setting up a redirect in the Zoho Mail settings or using a third-party service like a reverse proxy.
2. Third-Party Email Hosting: The user can consider using a third-party email hosting service that supports custom domains. This will allow them to manage their email hosting independently of Zoho Mail, but it may require additional setup and configuration.
3. Paid Plans: If the user requires more advanced features or support for multiple domains, they can consider upgrading to a paid plan, such as the Lite or Premium plans. These plans offer more features and support for multiple domains, including custom domains.

In summary, the user needs to follow the steps to add their custom domain to the free plan, which involves signing up, adding the domain, verifying the domain, and configuring the DNS. If these steps are not possible, the user can consider alternatives such as setting up a redirect or using a third-party email hosting service.

What are the limitations of the Zoho Mail free plan for custom domains?