What are you doing when it opens?

If it's clicking on any sort of link, set another mail application as default.
If it's when you open a browser, check your homepage or open tabs aren't trying to open mailto: links.
If it's opening automatically on boot for some reason, use the Autoruns tool to hunt down the issue.

Or, install FreeBSD on your desktop as I did in the past week. That's got rid of everything Microsoft :)

SirHumphreyAppleby:

 

install FreeBSD on your desktop as I did in the past week. That's got rid of everything Microsoft :)

 

Best fix right here

Try

• Start
• Settings
• Apps
• Default Apps
• Email
• Choose an application

If you want to be a bit more hardcore, you can also try these

• Windows PowerShell Console

   

  • Get-AppxPackage | Select-Object -Property packagefullname | Sort-Object -Property packagefullname
    {Find name of Windows Mail package}
    Remove-AppxPackage -Package <insert packagefullname for Windows Mail>
    
    
• Windows PowerShell Console with admin rights

   

  • Get-AppxProvisionedPackage -Online | Select-Object -Property packagename | Sort-Object -Property packagename
    {Find name of Windows Mail package}
    Remove-AppxProvisionedPackage -AllUsers -Online -PackageName <insert packagename for Windows Mail>