It's quite obvious for anybody who needed to concentrate on something
and was disturbed by an email and afterward couldn't get into the work
anymore, but here's a study that shows - using a simple task, the Tower
of London / Hanoi game - that the "new mail" popups disturbs and slows down.
http://www.esrcsocietytoday.ac.uk/ESRCInfoCentre/ViewAwardPage.aspx?awardnumber=RES-062-23-0101
(My) Conclusion: make the notification as unintrusive as possible, and
disable it per default.