Hi,
Don't use POP3 or IMAP; instead deliver the messages to a command. For example, GNU Mailman is mailing list software which pipes each incoming list message to a Python script. That command can then perform the necessary processing.
I don't know what you had in mind, but if the messages can be handled independently, it's easy to add new machines. Just duplicate the config on another box, and create an MX record for it.
Yep don't use POP/IMAP to fetch messages. Use a script as said Michael.
You can let Postfix handle the queue if there are more mails than your script can process. Or write a script that handle his own queue to avoid 'locking' postfix during delivery.
In all cases an architecture 'online' is better than 'polling' via POP/IMAP...
For scalability you can use multiple MX and/or spawn multiples threads on the delivery script.
Hope that helps.
Regards.