It is not entirely clear what your problem is and what you are trying to do.
If you have a BTYahoo email account you are already on the BT servers. It may be that your email client/app is using the Yahoo server settings which are no longer accepted so do you mean you need the settings for the BT servers so that you can set up an email client/app. If you do the BT settings are here
What do you mean when you say you decided to switch back to BT. Do you mean you were not a BT customer but was still using a BTYahoo email account but now you are a BT broadband customer and if so are you trying to have your "old" BTYahoo accounts associated to your "new" BT Broadband account?
If so have a look at this link.
Have you set up a BTID so that you can access your MyBT and your "extras" which includes any email accounts that are associated to your broadband account.
If none of the above is correct please try and clarify what your problem is and what you are trying to achieve.