> As always Paul you come up with the solution. I can now send emails. It is strange as the sending address should be the same as the address setup on the One.com site.
BATV changes the sending address (in the SMTP envelope, not the message itself) so that it is unique and cryptographically generated. Then, if you receive bounce messages to an address which is not a valid BATV-generated address, it knows that the bounce message was caused by a message that you did not actually send, so the bounce message will be silently discarded as it is backscatter caused by address forgery.
What could happen is that spammers send out thousands of emails and forge your email address as the sender (which is trivial to do). Then, when a large portion of those spam messages bounce, *you* would receive thousands of bounce messages for messages you never sent. BATV prevents that happening, but because your ISP only allows your exact email address as the envelope sender, you cannot use BATV, so you run the risk of this "backscatter" causing problems.