Well, there are two ways to tackle this:
1.) Send all orders to a catch-all account and pipe the email to a script
2.) Manually open the mail folders and pull out the information you need
I see #2 as an invasion of privacy (seeing as how the system then can read any and all email ever sent / received). #1 is your best option. Basically you'd want to create a new email account (call it "orders@domain.com") and then you'd instruct all sales personnel to now send all orders to the new address. This address would do nothing but push the email it receives to a php script that then reads the contents (and any attachments) and adds puts them in the PM box.
Now, separating the messages to go into the PM box can be done a few ways. One is that you just drop a message to all office personnel about the new order. Another is to add a "tag" to the body of the email, say an "ATTN:" line, in which if it's there, it will be forwarded to that persons PM box (searching by first & last name of course, or email address, or username... etc). And a third way is to just take it one step further and just insert the order yourself. Require your sales personnel to format the email in one way, and then if it's not formatted properly, just kick it back to them from the PHP script.
This is no easy task to undertake. You have to make sure paths are right, and learn how to deal with MIME mail, plain text mail, HTML mail, and other junk. And if you're allowing attachments, you'll have to learn how to strip those as well. But if you can devote some good time to it, it's a worthy investment 😉