As I said, it is probably a BT server problem. The best advice when it happens is to do nothing, especially do not change passwords or delete the account from your device as that can cause more problems than it solves.
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. As you have found you could log on there is obviously nothing wrong with your password or account hence the advice to do nothing.
If the problem does not resolve its self after a day or two or in your instance where you have changed your password on your device you should stop your devices from checking for email. The easiest way of doing that is to turn them off or add a couple of characters to the email address so that it is not checking your account.
Once you have done that, change your email password again and check that you can access your email via webmail (on a browser).
If that works you should wait at least an hour to allow the BT servers to sync and then add the new password to your devices, remembering to remove any characters you may have added and again check it works. If it does then move onto the next device if you have one.
When entering the new password in the settings, make sure that in the Outgoing server you do not use the "use same as incoming" for the email address and password. Enter them manually.
When you change your BT password on an iPhone/iPad/Apple device it may only change for INCOMING mail.
To change/check the OUTGOING mail password you have to go into Settings; then scroll down to Passwords & Accounts or on older devices Mail, Contacts, Calendars. From there select the BT email 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 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 have deleted the email account and you are setting it up again 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.
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoin...
