What are the risks associated with allowing employees to auto forward e-mail messages to outside accounts, such...
The biggest problem is that corporate e-mail messages are now outside of your *physical* control. They now reside on outside servers run by other people. They are on outside e-mail machines that have different security policies. They are accessed using passwords that do not have to confirm to your password policy. They sit on outside servers that get backed up. Now those backup media have your (perhaps) very sensitive corporate proprietary information. It is out of your control forever.
These are not good things.
For more information on this topic, visit these other searchSecurity resources:
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Executive Security Briefing: E-mail security: Defending the server
Chat Transcript: E-mail Security