I set up Thunderbird syncing with my Fastmail account some time ago - my Fastmail account has a bunch of email accounts synced to it which I can send and receive from. When I set up Thunderbird via the wizard, it automatically seemed to enable me to send emails via any of the accounts associated with my Fastmail. I recently added another email account that syncs via Fastmail, however I don't have the option to send via this email address in Thunderbird. How can I get the ability to send via this new email address?