Sounds like your server setting is wrong did you enter manually?
Check you do have the correct settings as per this link
This may be relevant if you have changed your password recently.
When you change your BT password on an iPad/iPhone it may only change for incoming 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.
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
If you tap on the outgoing mail server it shows that it is 'on'. You have to tap on that Primary Server in order to find where the Outgoing password is hidden so you can update it. Once you have found it make sure that the correct password is entered.