Problems with POP
Problem
High 'forward' traffic is the result of using POP for mail retrieval on a mailbox that is also used interactively by a person. What happens is that for interactive mail (experienced in Gmail) that mail sent TO a customer FROM the mailbox, and added to the Sent folder, also gets retrieved by POP, and is then rejected and forwarded. Reviewing the addresses will show a catchemail Filter match failure, the TO: address not being valid (the customer address).
Solution
The solution here is to stop using the mailbox interactively, or use IMAP.