CP is now the preferred mail exchanger for btinternet.com (again)
btinternet.com MX preference = 10, mail exchanger = mx-bt.mail.am0.yahoodns.net
btinternet.com MX preference = 1, mail exchanger = mx.bt.lon5.cpcloud.co.uk
Also, a CP server is now being returned as one of the addresses for mail.btinternet.com
So it looks like BT are dipping their toes in the CP water once again.
If you do happen to resolve mail.btinternet.com to 188.8.131.52 (or you force it via the as-yet unofficial server smtp.btinternet.com) then no check is made to validate the "from" address is actually yours and you can send "from" any address you choose. Perhaps this is to be the new intended behaviour.
Edited by Oliver341 (Wed 18-Sep-13 15:35:05)