That's not the right approach. The default is for ONE account and is used when you aren't working in other accounts. That is, if you are reading a message in account xxx and click 'new message', thunderbird assumes you want to send from your xxx account. If you're not lighting any particular message and click 'new message', then thunderbird uses default. So, the default only works when thunderbird cannot sense your preferred account. When you're in another account, it assumes that is the one to use.

Thanks David,BUT how do I resolve it????? Thanks Darryl

I am not sure I understand. What I posted is how it works. You cannot set thunderbird to default to one email account, regardless of what message or account is being viewed.