cannot delete a contact from safe senders list in Outlook web app

I have a contact i want to delete as anytime i try to modify my safe senders list, i cannot save, because i get a pop up:

Junk e-mail validation error. Value: 'robert.kennedy@hp.com.. the contact is not in my contacts folder, but this contact is preventingme from modifying.

i have modifed my max safe sender list from the default value of 1024 to 4096 via exchange powershell command, still no luck.