In March, the firm's Retail tentacle applied restrictions that meant customers who do not use an "@btinternet.com" address for outgoing mail must verify it to avoid a 553 SMTP error.

Obscure instructions on BTYahoo! co-branded pages made it frustrating for some customers to comply with the new regime. Others were angry that not enough warning was given.

BT has now decided to clarify the process before giving it another whirl. A statement released by the firm said:

"The email security enhancements we introduced earlier this month, although only affecting a minority of our customers, had a greater impact than we had expected.

This is an important security enhancement needed to prevent spam and phishing abuse on the Internet however we rolled-back recent phases of this work as soon as we realised the impact it was having on customers and this will have cleared most of the recently-experienced 553 errors.

If you have use a different email address in the "From" field of your email client to that of your BT Yahoo! email address then it can still be used as long as the email address is not another BT or Yahoo! email address."

BT have said that it would try again "once we are satisfied the customer journey is as clear and simple as possible". So when you receive an error sending emails, you may wish to check your settings to ensure that your email address is a valid BT email address...