Hi
I haven't been using Spartan but I just tried it and is did play the video OK.
I then sent an email to myself with a link to a video in it and opened it using the Windows 10 mail client.
When I opened the message in Windows 10 Mail, and clicked on the link it opened the video in Google Chrome.
My guess is that you don't have any settings that tell the Mail Client where it should open web links.
I looked and I couldn't find any settings that related to that.
I have no idea how it decides other than looking at what you have set as your default browser.
I Chrome is my default.
What happens if you copy the link from the mail message and paste it into the Spartan address bar?
If that works then Mail must not know what it's supposed...