Official comment
You can restrict the login for any user in your account to one or more IP addresses using email verification. To configure this:
- Click your email address at the upper right of the account.
- Click Administration.
- Click on IP Address Restrictions under User Management.
- Select the Enable IP address restriction check box.
Now, whenever someone tries to log into your account using your User name a "Restricted" notice will pop up with a button to authorize that IP address. Clicking that button will send an email to that User; responding to the email from the same IP address will validate that IP address.
Note this will happen the first time you try to log in after selecting the Enable IP address restriction check box.
You can edit the allowed IP addresses by clicking your email address at the top right of the account, selecting Manage Users and editing the User.