I think it is a matter of setting the predefined association in Setting -> Predefined App but I don't know the file extension type. I have already found set to Outlook 2016 the following file extensions: .vcs .vcf .pst .oft .ics .hol .eml
... and I did not find any association with the Mail app even though it is the one that it is open when I click on a notification of a new arrived email from the Notification Center.
Anyone can give me some advise??