Are you a BT Broadband customer and was the email account set up on that broadband account or are you paying for your email through a BT Premium email account?
How are you accessing your emails. Is it on a web browser (webmail) or is it with an email client or email app and if so which one?
I have have had bt broadband for years.
This has only started today. I am receiving but not able to send.
My btopenworld.com account has been set up for years.
I am using a stock android mail app to get mail.my Gmail account is working with no issues
Are you on the BTYahoo email system or the BTMail system?
If you are nor sure you should log onto your email account and look at the BT logo. If you see Yahoo mentioned it means you are still on the BTYahoo system.
BT are in the process of moving BTYahoo email accounts onto the BTMail system.
Have you checked your settings as per these links.
You should log onto your MyBT and go to "Your Products" > "Email, Manage" and check that your email account is showing there and what status it is showing along with checking to see if it shows as a "Basic" email account.
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, as in your case, there is obviously nothing wrong with your password or account hence the advice to do nothing.
If the problem has not resolved 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 device 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 device, 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.
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.
I have spoken to CS and it could be to do with the change over. They have had several reports. As they do them in blocks it could be something gets knocked off then put back on as this occurred a couple of months ago.