Hi scotland09,
Our spam prevention software checks each users' IP address against a database of addresses that have been associated with reported spam and other malicious behaviour. In your case, the IP address that you are using has at some time recently had spam/phishing emails (about NatWest and other banks) sent to others from it. It may have not been you or anyone else at your home, as sometimes IP addresses can change on a daily basis, so it may have been someone else entirely. Often this is not intentional spam but caused by a virus on a device at the address.
If your ip address does not change and you continue to have problems with needing to whitelist, you should have all of your devices scanned by virus detection software. Otherwise there is nothing further to worry about.
I hope that helps.
Leah