I have exactly the same problem , and no matter what I do, the iPad will not accept the account.
I have followed all of the instructions to the letter, and nothing changes.
I have other “@btinternet.com” accounts on the iPad, but this particular account will not upload.
I have this account on my iPhone and another iPad, but will not upload on my new iPad.
It was o.k. until I did a factory reset, and now will not upload.
I keep getting the message that my username or password are incorrect, but I know that neither are incorrect, as I use them to sign on to “My BT”.
I would be grateful for any input.
The first thing to do when it happens is check to see if you can log on to your email account via a web browser. If you can there is obviously nothing wrong with your password or account.
You will have deleted the email account during the factory reset and now setting it up again is not happening because the BT servers are not accepting the details. You should stop your iPad and any of your other devices from checking for email on that email account.
The easiest way to do that is to turn the devices off.
Once you have done that, change the email accounts password and after doing that check that you can access your email via webmail (on a browser).
If that works you should wait at least an hour and then add the new password to your iPad and again check it works. If it does then move onto one of your other devices and change the password to the new one. If that works move onto the next device.
If the above does not resolve the problem, when you change your BT password on an iPhone/iPad/Apple device it may only change for INCOMING mail.
To change the outgoing mail password you have to go into Settings; then scroll down to Password and Accounts or on earlier iPads Mail. contacts,calendars. From there select the BT email account and select the account where it is listed under IMAP if it has been set up as an IMAP account.
You will find the details for the account and can change the password for the Incoming server there, 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.
If you are resetting up the email account on your device you are best to set it up manually, using "other" and ensure that you are using the correct settings as per this link.
Silly me !
Because my username for my bt account is my “@btinternet.com” account and the password allows me into “mybt” I wrongly assumed that the password was the same for my email account.
It is not - I must have changed it at some point.
Thankfully I still had a note of previous passwords.
Won’t make the same mistake again !