The list of mailboxes and/or public folders used to run manual/scheduled scans in Symantec Mail Security for Microsoft Exchange (SMSMSE) is empty or incomplete when attempting to configure or run a manual/scheduled scan.

In a debugview log (Section D of How to Obtain Debug Logs for Symantec Mail Security for Microsoft Exchange (SMSMSE)), a message similar to the following is observed during service startup:

AVFMSESJM(<process ID>)[<thread id>] <datetime>:
..\..\..\src\Server\SAVFMSESJM\TExchDataProvider.cpp(268) :
Listing all user mailboxes on this server/cluster

SAVFMSESJM(<process id>)[<thread id>] <datetime>:
..\..\..\src\Server\SAVFMSESJM\TExchDataProvider.cpp(1195) :
Setting current Exchange server: <server name>
 
EMSStoreAccess: Using Rpowershell mechanism to get mailbox DB's

EMSStoreAccessE2k10_E15 Exception: Setting the execution policy for current user failed. 
Windows PowerShell updated your execution policy successfully, but the setting is overridden by a policy defined at a more specific scope.  Due to the override, your shell will retain its current effective execution policy of "<policy value>". Type "Get-ExecutionPolicy -List" to view your execution policy settings. For more information, please see "Get-Help Set-ExecutionPolicy." 
EMSStoreAccess Error: Failed to load Exchange Powershell snapin through Remote Powershell. Failed to Mailbox Databases 

SAVFMSESJM(<process ID>)[<thread id>] <datetime>:
..\..\..\src\Server\SAVFMSESJM\TExchDataProvider.cpp(1240) :
No MailBox Store(s)

SAVFMSESJM(<process ID>)[<thread id>] <datetime>:
..\..\..\src\Server\SAVFMSESJM\TExchDataProvider.cpp(293) :
No mailbox located, creating empty masterlist

A Group Policy Object (GPO) is blocking SMSMSE's attempt to run a powershell script to query Exchange for the list of mailboxes/public folders.

Configure the "Turn on Script Execution" GPO to allow SMSMSE powershell scripts to run:

To configure the GPO:

1. On the Domain controller, open the group policy manager (Start -> Administrative Tools -> Group Policy Management)
2. Expand the Forest, Domain, and Domain controller, and "Group Policy Objects" folders in the navigation pane.
3. Right click the "Default domain policy" and select Edit.
4. In the resulting "Group Policy Management Editor" window, Expand "Computer Configuration", then "Policies", then "Administrative Templates", then "Windows Components", then "Windows PowerShell".
5. In the right pane, right click the "Turn on Script Execution" policy, and set it to either "Allow local scripts and remote signed scripts" or set the policy to "Not Configured". Any other configuration of this policy will result in further errors.
6. On the Exchange server, open the a command prompt (Start -> Run, cmd) and type "gpupdate" then press Enter.
7. Restart the Symantec Mail Security for Microsoft Exchange service.

After a short delay (up to 5 minutes) the list of mailboxes should now display successfullly, and manual/scheduled scans should run successfully.