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seems to be a issue if using cached mode in outlook

https://learn.microsoft.com/en-us/outlook/troubleshoot/user-interface/email-not-be-delivered-scheduled-time Sounds similar, but not exactly the same...

Yes, we have had a few cases about that too. I've solved it for now by disabling exchange cached mode. Hopefully that will solve two things, the mailto links will work, and users will start using outlook and teams on their endpoints.