As the Event 15006 logged, please verify that the hard disk contains more free space than the threshold that has been configured for the Transport server. Please refer to: https://technet.microsoft.com/en-us/library/bb201658(v=exchg.160).aspx
If in the case, you cannot make the necessary disk space adjustments. Then we can choose to change the location of the queue database to a disk drive with sufficient free space. It can be done by following the following steps:
- Begin by terminating the transport service
- Go to the original queue folder (the one containing all the queue database and logs) and copy the files to the new desired directory.
- Open notepad and open the file C:\Program Files\Microsoft\Exchange Server\V15\Bin—EdgeTransport.exe.config
- Edit the following Keys:
<add key=”QueueDatabasePath” value=”C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data\Queue” />
<add key=”QueueDatabaseLoggingPath” value=”C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data\Queue” />
Note: The paths above need not be identical.
Hope it helps.