In a nutshell: don't do it!
Employees have successfully relied on the federal Stored Communications Act (SCA) to make the argument that employers are liable for actual damages, statutory damages, equitable relief, punitive damages, and attorney fees resulting from unauthorized personal email access. The SCA provides these penalties for whoever is not the ISP or the intended user and engages in the following conduct:
(1) intentionally accesses without authorization a facility through which an electronic communication service is provided; or
(2) intentionally exceeds an authorization to access that facility;
and thereby obtains, alters, or prevents authorized access to a wire or electronic communication while it is in electronic storage in such system shall be punished as provided in subsection (b) of this section.