Greetings Miguel,
The Cisco IronPort Email Security Appliance does not set a limitation per users. When sales provides specifications they do sometimes refer to the number of mailboxes but this is generally used to determine one of many factors that involve load on the appliance for capacity planning purposes. The ESA performance and capacity is measured in throughput. You can get an overview of your appliances current status by issuing the command 'status detail' in the CLI. This data is updated every 60 seconds.
If you would like to see the users that are currently logged into the system you can issue the command 'who' in the CLI.
Christopher C Smith
CSE
Cisco IronPort Customer Support
