I've moved this out of the Consumer section because it appears to be a problem that's been raised before by users in the Business section.
It's now in Business / Email and Web Security / SaaS Email Security, where someone should be able to advise you what to do.
You've already done what you can from your side. The message scored high for spam and was denied based on the recipient's designated action for the level of scoring it reached. The saas_falsepositives mailbox is the best avenue for non-customers, otherwise the recipient will need to open a support ticket with the support team that works with their installation and submit the false positive that way.
See the following for more information: