I have been told by OZ lotteries that your emails are being soft bounced with the error "Email is not delivered because the mailbox is full or for other reasons"

If your mailbox is, indeed, full, you should clear it out. It's up to you to manage your email to avoid coming up against your email provider's storage limits. Thunderbird can help you manage it, but it's ultimately your responsibility. Is this account set up in Thunderbird as a POP3 or IMAP account? If IMAP, when you delete messages in Thunderbird, they will be deleted on the server as well. You can also Move messages to Local Folders and they will also be removed from the server. If the account is set up as POP3, depending on your settings, you might have downloaded messages to your computer, but left copies on the server. That can result in you using your storage quota without realizing it. In my opinion, this is a poor way to manage email because it can lead to consequences like that, but it's entirely your choice. To answer your question more directly, the fix is to... - Use your email provider's web mail page to examine your Inbox and delete anything there you don't need; - Think about how the account is set up in Thunderbird and if it best meets your needs. If not, consider changing it.