Who has the right to read your e-mail? Old hash, but I need to ask.

A recent incident brought this up at work.
If someone could point me to a site that would help explain the in's and out's 
of the laws regarding the following -

Who has the right to read your personal e-mail.
E-mail created at work?
E-mail downdloaded at work?
E-mail stored on a PC at work? 

Thanks 
vwbug@iadfw.net