As has already been pointed out, if your password works on Webmail there is obviously nothing wrong with it and it is pointless changing it again and again.
The following might be relevant because you have changed your password.
When you change your BT password on an iPad/iPhone it may only change for incomimg mail.
To change the outgoing mail password you have to go into Settings; then scroll down to Mail, Contacts, Calendars.
From there select the BTinternet account and select the account where it is listed under IMAP.
You will find the details for the account and can change the password for the Incoming Mail Server there, but the outgoing mail server only shows SMTP and mail.btinternet.com
You have to tap on that Primary Server in order to find where the Outgoing password is hidden so you can update it with the new password.
If it does not have the correct password it will not authenticate and report an error.
Dear all, I have two iPhones and an iPad all working happily with BTYahoo (@btinternet.com) email. I changed the password on one of our 4 accounts using the edge browser on PC, all normal. I then needed to update the password on the apple devices as follows: Settings/Passwords and Accounts. 1. I selected the account and changed password, this didn't work message came back username & password incorrect. 2 I deleted the account and re-entered account data, same result username and password incorrect. I checked account by using edge browser on PC and Firefox on iPad all normal. Clearly the Apple world was getting confused so I deleted the individual e.mail account from all the apple devices. I then re entered the account on an iPhone again same problem username/password not recognised. Big Problem! SOLUTION I deleted account from iPhone so problem e.mail address was no longer on any of the apple devices. We went away for 2 days on mini break, on returning home I entered as a new account the email address etc in iPhone (settings/Passwords and Accounts/Other) ACCOUNT ACCEPTED! I then did same for other iPhone and iPad all working normally. Great relief! My conclusion is that there must be an account synchronisation delay between the apple email world and the BT Yahoo e.mail world. The fact that BT seems to have given up on the Yahoo iPhone app probably doesn't help! Anyway this definitely works!
Thanks massive help for letting me know this also worked for me