Last night I received the follow email from a customer…

Users in our domain.com domain are getting this error message when opening Outlook.  Outlook still appears to function as email flows in and out.

In looking around, it appears to be a Microsoft issue:

http://community.office365.com/en-us/f/148/t/263614.aspx

Do you have any info on this?  Or how to fix it?

Following the link the customer sent me, it does appear that this issue is happening for a number of different customers on Office 365.

Office 365 customers who manually install Office Pro Plus update 15.0.4649.1001 are receiving a pop-up security warning from an AutoDiscover failure.

When Outlook does an AutoDiscover search, one of the places it looks for information is https://domain.com/autodiscover/autodiscover.xml. When Office 365 customers do not have this record in place because they have migrate off of on-premises Exchange this updated version of Outlook is showing the security pop-up.

This issue does not affect Outlook functionality in anyway. Everything keep working as expected, but the users see a security pop-up.

This pop-up can be suppressed by adding the following registry key

[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\AutoDiscover]

"ExcludeHttpsRootDomain"=dword:00000001

Apparently Microsoft is aware of this issue, and a fix is forthcoming.