Unmark as spam

I send mail to a yahoo email user via planningcenteronline.com

At some point in the past they accidentally marked one of my messages as spam. They have since added my email, and the domain my email comes from to their whitelist, but yahoo continues to bounce the emails as spam.

Returned message: "Reason for return: hardfail - 607 - Not delivering to a user who marked your messages as spam."

The original email marked as spam is no longer in their spam folder, so they cannot go back and un-mark it as spam.

How can they un-designate my emails as spam?

Thanks for any help.