Explanation
This Warning event occurs if the available space on the hard disk is below the threshold that was configured on the computer that is running the Edge Transport server role or the Hub Transport server role. If the available space is below the threshold that has been configured for the server, messages are rejected by the Transport server.
User Action
to resolve this problem, verify that the hard disk contains more free space than the threshold that has been configured for the Transport server.
For More Information
if you are not already doing so, consider running the Exchange tools, which have been created to help you analyse and troubleshoot your Exchange environment. These tools can help make sure that your configuration aligns with Microsoft best practices. They can also help you identify and resolve performance issues and improve mail flow. To run these tools, go to the Toolbox node of the Exchange Management Console. To learn more about these tools, see Managing Tools in the Toolbox.
http://technet.microsoft.com/en-us/library/ff360633(v=exchg.140).aspx
Expanding The VHD
1. Shut the VHD Down in Hyper V
2. Delete any snapshots associated to that Virtual drive
3. Let the snapshots & VHD file merge.
4. Using edit disk, expand the size of the VHD
5. Mount the VHD & expand the hard drive
http://www.petri.co.il/expanding-virtual-hard-disks-with-hyper-v.htm