I am sure this is an old question, but I can't find the answer.
I am using Thunderbird 2.0. I am also on a volunteer ISP system therefore we have no email server service so we use gmail accounts. The problem is when people receive our email the from box says 'email@example.com' instead of our real address 'firstname.lastname@example.org' . This is confusing people we email as they reply to the gmail address instead of the address that gmail forwards to. I have tried gmail settings 'FROM BOX' and set the box to 'show this address' with our home email address not the gmail one. It does not work. thanks