Since some emails may have to be re-learned as spam or possibly a false positive, the original training data is stored temporarily for relearning. This is usually necessary, as most mail clients rewrite and even completely mangle the message when you forward it. Storing this information server-side ensures that all the data is retrained correctly. Each email processed includes a serial number to identify the signature. This is frequently referred to as the DSPAM signature and looks like this:

!DSPAM:3eb2c721141672274659!

DSPAM has a user-level option to embed the signature in the headers instead of the message body, however this will require the user to forward all spam as attachments (a feature not all clients have). Users may also opt to eliminate the DSPAM signature if they're willing to retrain using the Web UI. Finally, if you're using strictly IMAP or web mail, then you can eliminate the signature entirely and configure DSPAM to retrain using the original message.