It works fine on BT Webmail.
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 phone 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. If you can there is obviously nothing wrong with your password or account hence the advice to do nothing.
If the problem did not resolve its self after a day or two or in your instance where you have deleted the email account from your device you should stop your iPhone and any other devices from checking for email. The easiest way of doing that is to turn them off if you still have the account on your device or add a couple of characters to the email address so that it is not checking your account. If you have deleted the account and it is no longer on your device do not try and add it at this time.
Once you have done that, change your email password 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 servers to sync and then add the new password to your iPhone, 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 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.
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 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.
When setting 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.
Thanks. I have tried many of these suggestions.
i can only hope that it may correct by itself.
Perhaps it is worth trying another email client app?
I use Thunderbird successfully on my PC mainly because the BT server direct is painfully slow.
on my iphone and Ipad have always used the normal ios system which upto yesterday was fine.
Thank you Steve Brass and Distinguished Sage for all your help.
The problem i discovered this morning is that on the iphone and ipad in the "Imap Account Information" both the name and description showed up as the initials for my account.
When I changed the "Name" to "BT MAIL" miraculously I could send emails again.
Strangely my other accounts on the same system still work even though again they do not say "BT MAIL".
The Lord and Bt work in mysterious ways.
Where would someone such as myself, with very limited knowledge of such things, go to check my Imap account information?
The email app on my iphone has gone a bit bonkers since the most recent update and the majority of folders have disappeared.
Similar to your own issue Delta 1, there hasn't and isn't any problem with the other mail accounts on my iphone, just my main one and I now have to use the BT app to access my mail, which isn't exactly ideal.