The mail attribute will be populated in the campus Active Directory as of March 19th, 2018 whereas it previously had not been populated.
The mail attribute in AD.WISC.EDU will be provisioned by the Office 365 team to be the same as your primary Office 365 address.
You can change your primary Office 365 address please follow the instructions here.
For information on how this may impact your AD-joined applications, please consult your specific applications' documentation regarding how it handles the AD-delivered mail attribute.
Please Note: As this attribute can be changed we do not recommend using this as a key identifier.