Hi @Anonymous ,
To my opinion , the possible cause of this issue may be due to Network settings.
You could try the following steps to troubleshoot:
- Check the DNS settings of your Exchange server.
(Notes : Please ensure that the IPv4 value is the correct IP address for dc.And the altenative DNS server should be left blank.)

2.Use the following commands to check the messages failed at which step in message tracking log:
Get-MessageTrackingLog -MessageSubject "text1" -Sender "******@contoso.com"

3.Use the following commands to check if the messages are stuck in the queue:
Get-Queue -server e16

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