Few years ago during the litigation with National Labor Relations Board the question of working emails was raised. Employees were officially allowed by the law to discuss working conditions over emails (including oppression, unfair management etc). The case name is Purple Communications, Inc., 361 NLRB 1050 (2014).
Amazon employees live in fear. They are afraid to complain. They are afraid to discuss working conditions. Emails are monitored. It is illegal. Every case of retaliation by the company should be reported to the National Labor Relations Board.