I would expect it to use the default mail account email address. Is it using the default mail account email address? The default mail account is usually the one listed at the top of the Folder Pane unless you have used an addon to rearrange them. You set the default mail account in Account Settings. In Thunderbird right click on a mail account in Folder Pane and select 'Settings' Select the mail account one you want as default click on 'Account Actions' select 'Set as default' click on OK see image below as guide.