So I ran across this horrible problem this past week in doing an Exchange 2003 to Exchange 2007 migration. My customer used Exclaimer heavily for doing auto replies and for creating automatic signatures for the users within the organization.
After installing this app and then trying to configure the Captaris Rightfax Connector for Exchange 2007, I ran into an instance where when sending to an address formatted as:
Exchange generated an instance undeliverable message that it could not be delivered.
After working with the great folks at Captaris (Read as: some jerk on the phone that constantly spooke over you and did not really want to help anyways) and Microsoft, we discovered that Exclaimer re-writes the IMCEA address that's generated when sending to the FAX: or RFAX: address space, to the opposite case it needs to be. In other words, it takes IMCEA and makes it imcea.
Ready for the kicker? Exchange 2007 is case sensitive. The only solution was to uninstall Exclaimer and move on for now. The customer accepted the problem.
Just wanted to save you peeps some time!!