If you have received an email and you want to mark it as a "Safe Sender", in the list of your received emails you just need to tick the box to the left of the email address in the list then select "Safe Sender" in the menu bar at the top of the list.
If you have not received an email but you do know the email address that you want to mark as a "safe sender" Log onto your BTMail account then click on your username at the right hand side. Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Safe Senders" and enter the email address into the box and click "Save".
