Hey there judith7654,
Thanks so much for using Apple Support Communities.
We can help get you pointed in the right direction to getting your inbox sorted out. The first thing that we would recommend that you try deleting those emails directly on the BT webmail client. This can help isolate the issue, and is an important troubleshooting step to try. You can log in here and try deleting those messages, then check and see if they come back into your inbox on the webmail page: BT - Email Login Page.
If that resolves the issue, then you should be good to go from here.
However, if the issue still continues to occur in the Mail app on your iPhone, then we would recommend that you delete the BT account from your device and try adding it back from scratch. You can use this article to give that a shot: Add and remove email accounts on iPhone - Apple Support
We hope that this information is helpful.
Have a great day!