Pop up window "Send blocked" for external users inconsistency with M365 E5

Hello sumo83,

Few things to check,

1. Is oversharing pop ups configured properly? Refer to: Get started with oversharing pop ups | Microsoft Learn
2. Have you checked the outlook version used by these users for working and non-working?

I would suggest to troubleshoot this by starting off with a clean test, 

1. 2 users - one working and one non-working
2. Confirm both has the same outlook versions
3. Both has the same DLP policy and of top priority
4. Make sure over sharing pop ups is configured properly as mentioned in the above article.
5. Below option is set in DLP policy
   

Hope this helps!

Regards,

PI

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