I think it all been said above but here are my 2 cents :)
As an employee using company equipment and company infrastructure the company has the right to review my email. I have an ISP which I use for personal email. It was mentioned above that network admins do not have time to review email this is I think generally this is true. At my old job emails where considered private unless a complaint was logged with HR (to the best of my knowledge this never happened). I now work for a power company we are regulated and emails are occasionally presented as evidence in regulatory hearings -here I am much more paranoid.
That's my $0.02
-D
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Just because your paranoid doesn't mean someone isn't out to get you