Ever clicked on a mailto: link in a web browser that opens up a write window in Thunderbird? If Thunderbird is not running it has to have a "default" for the from. That is the most obvious and simplest option.

But if you are in say local folders and click write, Thunderbird also reaches out for the default from address to decide what to put into the write window as there is no "acount" that has the focus on the left folder pane. The same is largely true if you are using the unified folder view as the primary folder that has focus in not an "account" folder but a "virtual folder" with the emails from many accounts displayed together.

When Thunderbird chooses to default is almost a black art from a practical point of view, sometime you wonder just why it chose to use it on this occasion. The reality is whenever it is not clear what account is in focus and that can occur in some odd points for various reasons. It is however a good reason to check the from address on every email you send before you click send.

A good idea is to set the default to the account you will most probably be wanting to send mail from. Those with more than one account tend to have a "Primary" account they use for almost everything and that is the one to select as default.

Thanks for the explanation. That makes sense. I have 4 email accounts configured in TB and for some reason it had the third one (which I don't use much) as the default one. I changed that to my main one as well.