Hi @javad alamian ,
Could you tell me more details about your environment? Is it a DAG and what's the version of your Exchange server?
Also when does this error message appear, after sending a message or other? If so, will other users got the same error?
Please try these methods:
- Restart the MS Exchange Transport service.
- Use the following cmdlet to check if the database was mounted. Get-MailboxDatabase -Status | Sort Name | Format-Table Name, Server, Mounted
If there are not mounted databases, use this command to mount them and retry. Mount-Database -Identity DB1
Also if you could, please try restarting the Exchange server to have a test.
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