We are using Microsoft SQL Server 2008 (SP3) - 10.0.5500.0 (X64) Sep 21 2011 22:45:45 Copyright (c) 1988-2008 Microsoft Corporation Enterprise Edition (64-bit) on Windows NT 6.0 <X64> (Build 6002: Service Pack 2).
The architecture is a scale out deployment.
Whenever we subscribe to a report in SSRS and if the subscription returns no data, then SSRS keeps sending report email to the subscribed user multiple times. The interval is a little random. Sometimes it is every 15mins or sometimes 30mins. But, emails keep coming in.