After creating a Subscription for a report I noticed it was not actually sending the email error reported was:-
Failure sending mail: An error has occurred during report processing.

To Find out what is the actual error take a look at why abortion the SSRS Logs (usually located at):-
C:\Program Files\Microsoft SQL Server\MSSQL.1\Reporting Services\LogFiles

The Error we are looking for is something along the lines of:-
Error will appear in the latest log as:-
ReportingServicesService!processing!e!DD/MM/YYYY-HH:MM:DD:: e ERROR: Throwing Microsoft.ReportingServices.ReportProcessing.ProcessingAbortedException: An error has occurred during report processing., ;
 Info: Microsoft.ReportingServices.ReportProcessing.ProcessingAbortedException: An error has occurred during report processing. ---> Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: Query execution failed for data set 'DataSetName'. ---> System.Data.SqlClient.SqlException: A severe error occurred on the current command.  The results, if any, should be discarded.
Operation cancelled by user.

The cause is abortion options most likely due to the report taking too long to be created.  Therefore check your timeout seconds in the Report Manager:-
Http://YourReportServer/Reports
Click Site Settings (Top Left Corner)
Under "Report Execution Timeout" change to the following:-
"Limit report execution to the following number of seconds: 3600" e.g. 1hr