Mail user agents (MUAs) can be configured to deposit spam that makes it through the DSPAM filters into the "missed-spam" IMAP folder. This can be done either automatically (via the MUA's own spam filters), or manually (by saving the missed spam in the "missed-spam" folder.

One thing to note is that messages saved in the "missed-spam" folder must be saved with their full headers. This is because there is information in parts of the headers that are usually hidden that DSPAM needs to retrain the filters on the missed spam.

mutt

Mutt does not have it's own built-in spam filters. However, keyboard macros can be used to ease the moving of missed spam into "missed-spam" folder.

The following keyboard macro (in ~/.mutt/muttrc or ~/.muttrc) can be used to move a missed spam (with full headers) into a "missed-spam" folder that is elsewhere (ie. not by mutt) sync'd with the remote IMAP folders, and delete the original message from your inbox:

macro index,pager S "<save-message>[CAL mailbox folder]/INBOX.missed-spam/<enter><enter>" "move spam to missed-spam folder"

"[CAL mailbox folder]" should of course be substituted with the path to the local directory where your CAL mail is stored. This is known to work for maildir mailbox folders. It may not work with mbox folders. A similar technique could probably be used to move spam into the missed-spam IMAP folder if handled by mutt directly.