Do you mean that you have to dial up your old ISP to send your email?
If so, and assuming you're using Outlook Express, then from OE select
Tools
Options
and click the 'Connections' tab.
Click the 'Change' button (to modify these settings)
In the Dial-up Settings window, click on 'BT Internet Free Call' to highlight it, then click on the 'Set Default' button. Click the 'OK' button.
That should do it.