Older Versions of Exchange ActiveSync Clients Get the Bullet

Exchange Online will require email clients to use Exchange ActiveSync (EAS) V16.1 to connect from March 1, 2026. Email clients that use older versions of EAS won’t be able to synchronize with Exchange Online to upload outbound messages or download messages, attachments, and calendar items. There should be relatively few clients using an old version of EAS, but it’s wise to check.

https://office365itpros.com/2025/12/16/exchange-activesync-161/