https://www.bleepingcomputer.com/news/microsoft/new-windows-10-mail-app-update-lets-you-draw-your-emails/

Microsoft has just started rolling out a new update for Windows 10 Mail app that adds the highly-anticipated inking feature. This feature was previously being tested in Windows Insider builds, but Microsoft has added the inking support to the default Mail app in Windows 10 for everyone and allows users to draw and write on their email messages with a variety of tools. 

The latest version of Windows 10 Mail app is 16005.10325.20091.0 and it began rolling out sometime last week. The inking capabilities in Mail app should show up on all devices including the ones running Windows 10 April 2018 Update.