This morning I got a message that I needed to enter an access code because my computer (the same and only one I've been using) is not authorized to log in to this account. It says an email was sent with a code but after waiting for 10 minutes I have not received an email and can't get into my account. Why was this suddenly sent when it's the same computer I use every day, and why am I not getting the email? If you implement a security feature like this you need to let people know, and make sure the emails are sent promptly, especially if you're going to add on a 24 hour lock out to get a new code sent – that's just good interaction design. Is there any resolution to this? Is there a window within which users should expect to receive the email? If that's the case, you should say that in the message - "Emails may take up to x minutes to be received, please be patient."