The Mail App is missing in version 10.0.0. Is this intentional? I thought it would be grouped under Productivity,
I think the ownCloud version of the app requires a version lower than the maximum version of Nextlcoud.
However the Mail app was recently forked and is available here now https://github.com/nextcloud/mail. There isn’t an official version released yet, but there is a pre release available which works fine for me: https://github.com/nextcloud/mail/releases/tag/v0.5.3 (please be warned it’s prelease 
)
@ChristophWurst please add anything you miss 
I see it in my Nextcloud 10, however it’s marked experimental, and you need to turn on “Enable experimental apps” in the Apps app settings to see them.
Unfortunately, the ownCloud app stores resets the official status whenever a newer version of an app is uploaded. Since we recently pushed a new release, we’ve lost that status again.
The most recent release (0.5.3) is compatible with Nextcloud 10
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Yes, that did it. After clicking Apps, I used that littler gear icon on the bottom left to enable experimental apps. What threw me off is that I did not need to do that in my previous install and now I understand why. Thank you! @ChristophWurst @Bugsbane
Glad it worked out for you! 