I work for an internet web host. I have access to all files on our client's servers so that I can provide technical support. This includes our client's email inboxes.
There is nothing mentioned specifically regarding ownership of information on our servers in my company's Terms and Conditions. So who technically owns the email messages? Is it unlawful, in any way, for me to view those messages without explicit permission? I do so only when necessary to assist them with the problems that I try and solve as per my job duties. I abide entirely by my company's privacy policy, never giving out any personal information.
The authors and recipients of email have an expectation of privacy. If you violate that expectation you could be held responsible. If you need to view emails you would be wise to obtain explicit permission, and to document that permission.