Has anyone seen a folder in Mail called Yammer Root? I tried to erase but it is titled a distinguished folder and cannot be erased

Yep, I started to see this in our Office 365 tenant of more than 70,000 users.

(The link that Eric Root provided is not at all related to the OP's question.)

Microsoft has recently been rolling out updates to its Office 365 application, specifically for the Exchange Online and Yammer integrations.

The 'Yammer Root' folder is supposed to be hidden. I've confirmed that Outlook Web App, Outlook Desktop apps, and the iOS mail app do NOT show the 'Yammer Root' folder but, for whatever reason, the Apple Mail client on OS X does.

I've reported the issue to Microsoft - I doubt they'll have much to do or say about it since Apple Mail isn't their product or [insert another BS excuse].

I'll update with any relevant news.

https://support.microsoft.com/en-us/kb/2840541

Looks like this is on Apple to fix the Apple Mail client.

Microsoft responded as such:

My name is Purvi Mendon and I am from Microsoft Yammer Enterprise Team. I tried to contact you on 313-577-4698 at 4.24pm EST regarding the case # 616090991744953 which was created regarding "Yammer Root" folder is visible in Apple Mail client for OS X”, however you were not available.

Regarding the issue where users can see the Yammer folder in Apple Mail. During my research I found that this folder is available in mailbox but marked not to be displayed.

The PR_ATTR_HIDDEN MAPI property on this folder is set to TRUE by-default. This means the client should not show this folder and this is the reason why the Mac Outlook, Windows Outlook and OWA does not show this folder

ag: 0x10F4000B

Type: PT_BOOLEAN

Property Name(s): PR_ATTR_HIDDEN, PidTagAttributeHidden, ptagAttrHidden

Set to TRUE

This indicates that the issue is with Apple Mail. Apple Mail is not reading this property correctly and showing this folder. I would request you to speak with Apple support and get this fixed in Apple Mail.

ok so this is not a virus then? or my email accounts are not hijacked? I have 2 accounts hosted with Outlook.com and synced with OSX mail and I've never seen these sub folders show up before.. no fancy code slang please

Correct - your accounts are not compromised.

This is just an issue where Microsoft updated the mailbox folder structure and for whatever reason, the Apple Mail client does not observe the 'hidden' flags like it should.