Here are some common reasons why you're unable to send emails, along with their solutions.
Check the domain's status from the Zoho Mail Admin Console page.
If the status is "Yet to reverify your ownership," follow these steps to reverify.
Note: The DNS changes might take some time to propagate across the globe. Please wait for 24 to 48 hours and then try to check the email delivery status.
When your account storage exceeds the allowed limit, the account gets blocked, preventing you from sending or receiving further emails.
You can try deleting unwanted emails or upgrading to any of the paid plans. If you are already subscribed to a paid edition, you can choose to purchase and allocate additional storage space.