We are seeing this error since going to ColdFusion 10 enterprise. We have installed all the updates. Server is Windows 2008 64-bit with all the latest updates applied.
This appears to be very random, but it is causing some headaches with the mail spool. If 10 emails are in the queue, 9 will go out and 1 will go to undelivr even though all the parameters are the same. We move the file back to the spool and it goes through.
Right now I am dedicating a resource to monitor the spool so all emails go out (we send out hundreds per day from our app) but we need some type of resolution. Any help is appreciated!