I ran into an interesting issue yesterday at one of my customers.  They have two CRM servers and a forwarding mailbox for each server.  The setup looks like this:

We had an issue with the second CRM server, which just went into production, where inbound mail wasn't being sent to CRM.  Looking at the forwarding mailbox, we found that all of the mail had ended up in the Undeliverable folder.  After a couple of hours of troubleshooting and searching the web, I found this article from Guenter that provided the solution to our issue.

It turns out the CRM Email Router service was running under the security context of a user called CRMMail.  We had to add that user to the PrivUserGroup security group for the second CRM server so that it would accept the forwarded email.