This is definitely an issue, “Settings –> Postmark” is gone from multiple sites I manage.
When trying to access “example.com/wp-admin/options-general.php?page=ac_pm_admin” directly I get the common error “Sorry, you are not allowed to access this page.“
Using 1.17.2 of this plugin and latest WordPress
Seem like it’s something related to the permission check/nonce. Won’t allow the plugin to display properly