If you are configured with a Hybrid Exchange Configuration this pertains to you! Microsoft recently released an Office 365 Message Center Alert that states you need to run a command on your Federation Trust due to changes Microsoft made to the Online Organization ID Certificates. This update needs to be made by September 23rd!
The command to run is below:
Get-FederationTrust | Set-FederationTrust –RefreshMetadata
I assume this requirement is for all Office 365 Exchange Hybrid Deployments, but cannot confirm at this time. I strongly recommend that you run the command no matter what, as this command will not have any adverse effects if you are not affected.
Be sure to keep an eye on your Tenants Message Center, https://portal.office.com/default.aspx#MessageCenterPage