I use Thunderbird with two different ISPs (one with multiple email accounts), which require different outgoing mail servers. In the Outgoing Server (SMTP) Settings box, one of these is set as the default - i.e. in bold with (Default) after the server address. I can change which of the the email accounts is the default (and therefore its outgoing server) using the Set Default button to the right of this box, but I can find no way to deselect any of them from being the default. How do you do this?