I’m pretty sure searching for account attributes isnt a functionality of Search. I would recommend creating a placeholder identity attribute(adMail) and source the account attribute into it. Then refresh that user so it populates on their details page. Next search on the identity attribute like so "attributes.adMail:“specific email here”. Personally I always have an identity attribute dedicated to the mail attribute for an ad account, granted usually that populates the mandatory “email” identity attribute anyway.
If you dont need the identity attribute after this search then you can delete it.
Alternatively you can go to your AD source and download the account csv, put a filter on the mail header and search for the specific email you need.