Sorry, but from your posting I am unable to be positive as to which way you have 'migrated'. As indicated by murraymints, if you had a BT address then you will have to set-up a new e/mail account; or pay BT £1.60 per month to retain the original address.
It'd be useful to know what error it comes up with; but yes, you can't expect a service provider to continue providing you with an e-mail account if you are now with someone else. There are free independent e-mail providers out there who don't care who you are paying to provide you with Internet access.