I can’t send emails from my btinternet.com via Mozilla thunderbird

PLEASE BE PATIENT WITH ME. I am elderly, and am struggling greatly. Within the last week, I can’t send emails from my Mozilla account. I have a BT email address ie btinternet.com. When sending, it comes up with Login to server mail.btinternet.com wth username (my email address) failed. This is followed by Unable to authenticate to outgoing server (SMTP) Please check the password and verify the authentication method in account settings / outgoing server SMTP. I was on the phone to BT for three hours yesterday and they were so lovely and patient, they set me up wth a BT email app, however all of my folders with my vital bits and bobs are on Mozilla. Please could someone help me