Open Thunderbird and go to the Tools Menu then click on Account Settings. Then click "Add Account"

If running Thunderbird for the first time, it will prompt you whether to import from a different mail program on that computer or not. If you choose not to, it will automatically start adding an account.

Choose Email Account from the list:

Go through the wizard and enter your name and email address:

Server Names. These are mail.yourdomain so e.g. if your email address is john@mybusiness.com.au then the mail server is mail.mybusiness.com.au

And User Name. This is your full email address.

Complete the rest of the wizard.

Now we need to set the port for the Outgoing Server (SMTP) as some ISP's block port 25. Go to Tools menu then Account Settings, then click "Outgoing Server (SMTP)" and then the "Edit" button. Change the port from 25 to 587. Under Use Secure Connection, choose TLS and click Ok