Ok so I think I know why, incase anyone else is having this issue. The email is being sent to one of my email forwarders which I use every now and again to send myself reminders, in this case 23 times which using the Auto Learn gives it a score of 23 * -25 = -575. The message is a bit misleading because if says  xxxx@yyyy.com is the sender that has been used before not the forwarding address. If I delete the line in the SenderHistory_001.dat which refers to the forwarding address then the message "outbound message were sent to xxxx@yyyy.com in the past" disappears.

So I would say there is a small bug in the message as it is picking up the forwarding email as the senders address but the message refers to the actual senders address.

Is there a way to ignore certain email addresses on the auto learn sender section?