Q. Can an employer check the e mails of the people that work for him and suspects they may use his computers and paid time for their own private benefit?

A. Avnei Choshen (p. 568) mentions that Rav Chaim Valoshener zt”l instructed the secretary of the yeshiva to open all the students mail; to make sure that the bnei yeshiva would maintain their Torah-righteous level. This “heter” is widely used today by many yeshivos and educational institutions.
Horav Shlomo Miller’s Shlit”a opinion is that it is indeed permitted since the hired people understand that this may be part of the normal expected office procedure for maintaining proficiency

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a