Getting this error mostly when using outlook desktop app to send mail to shared mailbox. But I use outlook on the web, it will deliver successfully. I have tried some tips gotten online like toggling on/off focus group, but the error still the same. The error message is given below:
STOREDRV.Submission.Exception; failed to process message due to a permanent exception with message recipient 'recipientName' is not resolved. All recipients must be resolved before message can be submitted. InvalidRecipients Exception: Recipient 'recipientName' is not resolved. All recipient must be resolved before a message can be submitted.