We have learned today that some of the biggest Internet Service Providers in the UK have begun blocking outbound email through their servers unless the address is "verified" - here is a link to
BT's explanation We don't generally provide outbound mail services, although we do of course handle a lot of incoming mail for our clients, and have recieved a number of queries about this today.
I'm afraid there's nothing we can do to help with this - if your ISP won't let you send outbound mail then you may need to think about who you use as an ISP!