Merge branch '6332-mail_user_get_all_by_client' into 'develop'
Add API function mail_user_get_all_by_client, #6332 Closes #6332 See merge request ispconfig/ispconfig3!1599
Add API function mail_user_get_all_by_client, #6332 Closes #6332 See merge request ispconfig/ispconfig3!1599