You can check the current usage of your mailbox, which includes the storage used by the Zoho account and all the POP accounts in Zoho Mail. The usage details will be available in the Mail Accounts page in the Mail Settings.
The personal @zoho.com account users have a free space of 5GB allotted storage. The business users have the allotted storage, based on the plan and the storage allotted to them by their administrators.
When the mailbox exceeds the alloted storage, the incoming emails to the accounts are blocked and any POP accounts are temporarily disabled. You can either delete emails to create more space or purchase additional storage, to create free space in the account.
If you are an organization user, you can request your administrator to allocate more storage to your account. If you are personal user, you can buy more storage from the Mail Accounts page in Settings. Super Administrator can buy the storage units and allocate to the users.
You can literally have a bottomless mailbox with Zoho Mail. The add on storage is available in units of 10GB, 15 GB, 25 GB, 50 GB, 100 GB and 200 GB sizes. In case you have an ever growing mailbox and need more than 200GB storage, you can contact support (at) zohomail (dot) com with your storage requirements, for us to get back to you with the relevant pricing.
Suppose you choose to delete older mails, instead of purchasing storage, you can proceed with the deletion of email using empty folder option, available in right click. Once you have deleted the unwanted emails or folders eating up your space, click Empty Trash, for the storage to be recalculated. Further, if you need to have a back up of emails in your Zoho account before deleting, you may download it to a POP client. The configuration details for configuring your Zoho based account over POP is available in this help page. The only option available to back up emails in the POP accounts added to Zoho Mail is the 'Export email' option.
Once you have purchased storage or deleted emails, you can also unblock your account, using the Unblock Captcha link in the Mail Accounts page.
The personal users of Zoho Mail, by default have a free 5 GB mailbox. You can opt to buy more storage, based on your requirements.
The domain hosting users, in Lite plan, have the default mailbox storage of 5GB. In case the storage is exhausted, you need to upgrade to a paid plan, to get accounts with more storage.
Please write to us at support (at) zohomail (dot) com, in case you need any further details in this regard.
The organization super administrator can buy additional add on storage units, for the organization and assign it to any user in the same organization, in addition to the storage available in the base plan.