Microsoft is starting to block these from delivering mail to EXO, so they'll be forced to migrate soon.

Only for sending to EXO from on-premise if you're in a hybrid configuration. EXO will still happily accept it from external non-O365/tenant tied installations, however.

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It's a measure to force *o365* customers to upgrade their installs if they're running hybrid. That's it.

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" To make their position clear and attempt to convince those running old servers to upgrade to a supported release, Microsoft will introduce a change to the Exchange Online transport service to check the SMTP headers of inbound messages arriving in through inbound connectors configured with the ConnectorType set to be OnPremises. This limits the set of servers Microsoft will monitor and potentially block to those in hybrid organizations that handle email traffic to Exchange Online. "