On April 15 2016, the Office 365 TLS certificate will be renewed. This certificate is used by Office 365 to provide TLS encryption between Office 365 and external SMTP servers. The new certificate, which will help improve the security of mail sent to and from Office 365, will be issued by a new Certificate Authority and it will have a new Issuer and Subject.
This change has the potential to stop hybrid mailflow between Office 365 and your on-premises Exchange servers if one of the following conditions applies to you:
- Your on-premises Exchange servers are running Exchange 2013 Cumulative Update 8 (CU8) or lower.
- You’ve upgraded the Exchange 2013 servers that handle hybrid mailflow to Exchange 2013 CU9 or higher. However, since upgrading to CU9, you HAVE NOT re-run the Hybrid Configuration wizard (either from the Exchange Admin Center or via the direct download link).
If one of the previous conditions applies to your organization, hybrid mailflow between Office 365 and your organization will stop working after April 15, 2016 unless you complete the steps below.
Read more at source: http://blogs.technet.com/b/exchange/archive/2016/02/19/important-notice-about-certificate-expiration-for-exchange-2013-hybrid-customers.aspx