Update:
I’ve replaced the Logo picture by a smaller one (448 x 201 px instead of 5000 x 4000 px)
and the Notification E-Mail looks better in Outlook 2016 on Windows 10:
So problem definitely seems to come from the rendering engine which is used by Outlook on a Windows 10 device.
Is there any chance someone will check the XWiki.Notifications.MailTemplate and tries to implement an solution for Outlook on Windows?
→ Probably with <!--[if mso]>
statements?
→ I will also try to implement this in the Notifications.MailTemplate and inform you if it would work
Best regards & have a good start
Dolo