Can an Email be both delivered and in the 'Undelivr' folder?

We are running CF 9 and are having occasional issues with emails. We have seen two instances of an email both being sent successfully and also being put into the 'Undelivr' folder.

We discovered this when a client said they were missing an email (a different one). We moved several cfmail files in the 'Undelivr' folder to the 'Spool' folder and this client received one email for a second time.

They have forwarded us both emails and we can see the different delivery times in their inbox. We have looked in our logs and we do indeed have an entry for when it was moved to the 'Undelivr' folder as well as the successful send. No other errors have been found in other logs.

Has anyone ever seen this? I am also curious what exactly makes CF sense a problem is occurring other than possibly a lost connection to an email server. If cfmail tag had an error, can it be trapped? We currently have no special coding around the tag.

Any help is most appreciated.