If you receive the error "Mailbox quota exceeded" when you try to send mails, the issue is that the email account to which you are sending mails will be over the allowed quota.

You can either increase the quota of the mail account or delete the unwanted/read mails from the inbox. Also make sure that the mails you have deleted are not stored in the Trash folder. Remove the mails in the Trash folder too.

You can view the quota used by each account by logging into your cpanel:

1. Login to cpanel

2. Go to Mail > Email Accounts

3. It will show the used quota and the quota set for each email account in the "Usage / Quota / %" column.

4. Now you can delete the mails from the account which is using large disk space.