You can continue to use the Fresserve/Orange POP3 servers to receive emails via any ISP but you may not be able to use their SMTP servers. Many ISPs, incl. Orange, block you from using other ISPs' SMTP servers.
You need to use your new ISP's servers. Ask them or look it up on their website.
Also, beware, now you've left Orange you will need to reactivate the Orange email a/c on their website every 219 days. They should send you an email on that a/c warning you and telling you how to do it.