It's not your password that's causing the problems. It's the actual server settings. They're listed (unsurprisingly under 'SMTP settings') here:
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoing-and-incoming-bt-email-servers
However I simply wouldn't bother trying to muck about with them. As I've already indicated, it's far simpler to delete the account and then create it again.
To delete your BT account (assuming that you're using the default email app on your iPad mini) go to Settings > Mail > Accounts. [On some iPad models you need to go to Settings > Passwords & Accounts instead]. Tap on the name of your BT account. Tap Delete Account. Tap to confirm.
To re-create your BT account, see here:
https://support.apple.com/en-gb/HT201320