Have you ever dealt with Email Router issues before? In my opinion, the Email Router is one of the complicated components in CRM. There are two versions available, on-premise and online. To configure the router, you have three different ways to do it and it all depends on your own environment and your preference. If you want know how to configure the email router step by step, please refer to the MS CRM 4.0 Email Router Configuration Scenario.
Today I encountered another issue related to the email router. One of my clients upgraded to CRM 4.0 couple months ago, however they didn't upgrade their email router. So they are running CRM 4.0 and using the CRM 3.0 Email Router. Since CRM 3.0 used the forward mailbox option, all users had the forwarding rule deployed to their exchange profile to cc emails to the forward mailbox. Since CRM 3.0 not able to promote tracked emails to CRM 4.0, all of emails remained in the mailbox and the mailbox was full. Users were getting email error telling them the mailbox reached its size limit. I logon to the mail account via OWA and I saw more than 44,000 messages in the mailbox.
To resolve the issue, I had to uninstall the CRM 3.0 Email Router and then reinstall the CRM 4.0 Email Router. I also configured it to use the forward mailbox option after the install. I don't have to redeploy the forwarding rule because the old rule is compatible with CRM 4.0. If you would like to remove or redeploy forwarding rule, you may use the CRM 4.0 Rule Deployment Wizard (You can't use the CRM 3.0 Rule Deployment Wizard to configure the rule after you upgraded CRM to version 4.0, you need to use the CRM 4.0 Rule Deployment Wizard to do so). If you configure everything correctly, the email router will start processing all of the emails in the forward mailbox. It will not promote all of the messages in the forward mailbox all at once. It will process small amount of emails every 10 minutes to limit the resources it will consume in the environment. So to process 44,000 messages, it will take some time.
OK, I hope that this will help you just in case you encounter something similar to this.