It means the mailbox has reached its storage quota, so the server can no longer accept new messages for it. Senders may get a bounce until you free up space.
- Log in to cPanel and open Email Accounts.
- Find the affected address and click Manage.
- Under Storage Space, raise the quota (or set it to Unlimited if your plan allows) and save.
To actually reduce usage, log in to Webmail, delete large or old messages, and empty Trash and Junk. Remember your whole account also has a disk limit, so deleting mail also frees overall space. If your account is at its total disk limit, you may need to remove files or upgrade; open a ticket if you need help.
