Problems sending email from onpremises to Office 365 accounts in hybrid environment.
When configuring a hybrid deployment, you must use and configure certificates that you have purchased from a trusted third-party CA.
To check if the issue is related to the certificate part, please manually remove the previously created hybrid connectors both in the on-premises Exchange server and Office 365, re-run the HCW (Hybrid Configuration Wizard) to re-create these connectors using the new certificate, then check if the messages can be delivered.
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