Hi @姚黎忠,
According to your description, there is only one external user have the issue to send message to your group, so we could know that the configuration of the group should be correct. I suggest you follow these steps to troubleshoot:
- Try asking the other users who are from the same organization with this specific user to send emails to the group, and check if the issue persist. This can help isolate whether this issue is related to the whole domain or single user.
2.Check if there are any mail flow rules that may affect or block messages from specific senders or domains. In the Exchange admin center, go to Mail Flow > Rules.
- Try to ask the specific user to send the message using the email address directly instead of any autocomplete or cache info.