That is a security error, not from Thunderbird. You may not have authority to send or recipient's server is set to refuse, or you're using the wrong server. Or you might be banned from perceived spam or a blacklist. I encourage contacting recipient and your email provider.

'Incorrect greeting' usually means the authentication method for the outgoing server is wrong. The recommended settings are: smtp.telstra.com on port 465, SSL/TLS, normal password, User Name = email address.

https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings