I'd probably solve it like this:

• Export all of the headers into a headersfile on an offline machine
  
  
• Run a script on the offline machine that processes only one day's headers at a time. When a day has been completed, it should be removed from the headersfile and the aggregate counts for each email address should be inserted into the database Aggregates table.
  
  
• The Aggregates table will have three columns:
  
    email_address, date, rawcount
  
  
• email_address and date comprise the composite key for the Aggregates table
  
  
• Since a record is not generated if a user hasn't sent email that day, superfluous records won't be inserted into the Aggregates table
  
  
• This script can be run on the offline machine in a controlled and limited way until all dates have been processed
  
  
• From then on, only the current day needs to be processed/aggregated
  
  
• You should then be able to efficiently query reports from the Aggregates table
  

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