With this new feature, enterprise users can remain active in Yammer without switching away from the Outlook experience. They can have the full Yammer thread and reply, like, vote, attach files, share gifs and even watch videos right inside the Outlook inbox.
If you do not want to see the Yammer conversation in email, you can turn off this on a per-email basis by selecting “Hide Yammer Conversation” at the bottom of the email.
Source: Microsoft