I don't think you need TBird in this case. I did a bit of digging and found this article from iinet https://help.iinet.net.au/iinet-email-messaging-company Long story short, the entirety of your iinet account is going to migrate to the Messaging Company and as long as you follow the steps in the link above you shouldn't lose any data. Afterwards, if you still wish to use TBird then it's as simple as signing in with your usual email and password. Let me know if that answers your question and also if you continue to need help but remember you need to migrate your account first because TBird isn't a domain but a client.