Reroute all WordPress mail through Postmark to increase email reliability.
Because sending email with your web server is horribly unreliable. While this might not be a problem when all it emails is some comment emails, but when you start relying on emails from your WordPress install for inquiries, order forms or transaction emails, you really should make 100% sure these emails arrive. Postmark makes that part easy. You might want to read the authors posts about email reliability and sending reliable email with Postmark.
There are definitely other choices of email parties out there, so far the author has just used Postmark and loves its simplicity.
By comma separating the from addresses in the wp-config:
Yes, simply copy the contents of the plugin directory (so not the entire directory itself) into your
wp-content/mu-plugins/ directory. This is the main reason why the plugin doesn't have an admin screen; it's meant as a drop-in functionality for sites.