The Difference Between Piping and Polling

Jeff Standen explains about "The Difference Between Piping and Polling"

Piping (a.k.a. Pushing)
With the piping method, e-mail is delivered to an application in real-time at the cost of server resources and redundant application overhead.
Pros:
It’s fast. E-mail is delivered to the helpdesk proactively and in real-time.
It’s modular. The relatively expensive operations required to convert a raw MIME e-mail message into plaintext with usable file attachments can be offloaded to a speedy, specialized application. In previous versions of Cerberus Helpdesk, e-mail processing was handled by a parser written in the “C” programming language.
Cons:
It doesn’t scale well. Each new message is processed concurrently, each costing the overhead of the application starting up, initializing resources, processing, and then shutting down between each handled message.
It can bounce/blackhole messages. Since mail is delivered in real-time, the mail server is generally looking for an immediate clear-cut success or failure response code from an application. That means if you’re upgrading, tweaking, rebooting, or otherwise taking your helpdesk offline for a few minutes, new mail is bouncing for later retry. That’s normal behavior. However, if your helpdesk is accessible but in a broken state (e.g. halfway through an upgrade, having database issues) then it’s possible for mail to be accepted but not processed.