Each appliance would be a little different based on the expected mail processing, throughput for your environment/domains... and then throw in which processes you have turned up (IPAS, AV, VOF, etc.)...
Typical C370 (running 8.0.1) should be able to handle:
1. ~18 +/- recipients/sec
2. average workqueue ~ 462
3. average CPU utilization of ~ 91%
The #s vary, again, based on what you have enabled and licensed. You would be well suited to open a dialog with your Sales Ops/Account team, as they have means to determine the proper numbers and outcomes for your environment.
I hope this helps!
-Robert
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