The Outlook developer team rolled the app back to a previous version because some users were experiencing problems. Gabe Aul said they're working on getting it added again soon.
It's not the build; it's just the Outlook Mail app. Gabe Aul's Twitter account suggests that they are doing some bug fixing, that's why this feature has been pulled out in an app update. Expect a succeeding app update that will bring back this feature.