Not just secure from outside forces, but also secure from employees?
In a recent survey published by Help Net Security, approximately half of all employees said they would take data, including customer data, when leaving a job.
This has major implications for ESPs, where employees have access to customer data and mailing lists. There are at least 2 cases that I am aware of where employees have walked out of a company with customer mailing lists, and I’m sure there are other incidents.
ESPs should take action to prevent employees from stealing customer data.

I’ve had way too many addresses leak from large reputable companies to spammers, presumably because someone at at ESP stole them. It’s a big problem.