Having issues sending mails to UU admissions

I've been trying to email admissions regarding financing and all attempts have been met with a "Delivery Delay" email. Has anyone faced this? What can I do to rectify this?

This was the message from server: 450 4.7.26 Service does not accept messages sent over IPv6 [2607:f8b0:4864:20::a2b] unless they pass either SPF or DKIM validation (bad or invalid public-key record)...

unsure what to make of this :/