You will have to ask Koolbear about that but I think if you look closer at the header info you might be able to figure that out on your own. If I'm not mistaken the DBB is only using the gmail address as the return address.
phpBB doesn't actually have the mail engine embedded, IIRC. It's reliant on whatever PHP supports. Check the PHP docs at php.net, you might be able to hack TLS support in yourself if it's merely not present in the phpBB setup options.