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At present the GetDomainStatistics function only appears to return bandwidth usage in respect of emails sent/received but it does not include traffic as found via the internal reports. It is possible for a user to use substantial amounts of bandwidth (particularly with the Outlook 2013 IMAP 'feature' which causes daily full syncs) which we cannot capture via the API and hence bill customers for.
Could GetDomainStatistics be extended to include all bandwidth (i.e. items sent/received plus that consumed collection mail) or another function be added that includes this? As mentioned the data is already collected, it just needs exposing via the API.