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Some days ago Microsoft a new roadmap for supported Exchange szenarios in hybrid environment considering mailflow traffic. There are some news about the so called Throttling dates (when Exchange Online begins to reject some messages from older Exchange OnPrem versions). See the original post and the roadmap here.
This behavior is communicated already in March 2023 see this the original blogpost.
As described above, Microsoft starts with Exchange Version 2007 as oldest supported version in Hybrid environments. This dinosaur will be reported on 23th of June 2023 (Report), July 2023 throttling is beginning (Throttle)and August 2023 mails from this version will be blocked (Block).
See the graph Microsoft designed to communicate the changes.
One interesting fact is that the movement to the latest Exchange version is quite fast. Exchange 2010 is blocked in November 2023 (I’m supporting this step!). Exchange 2013 follows on February 2024. You can only advise companies with older versions to upgrade to the latest Exchange System.
Unfortunately there is no newer alternative to Exchange Server Version 2019 so the classical Hybrid environment officially ends on May 2024 (todays view). Either there will be a newer version or Microsoft provides another solution for Azure AD Synced environments with Exchange Management. There is a pretty interesting article (german only) about Exchange Online Provisioning. It gives a short overview about the methods to edit and create Mailboxes with and without Azure AD Sync.