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 its own built-in spam filters. However, keyboard macros can be used to ease the moving of missed spam into missed-spam folder.

mutt on a CW (via straight IMAP)

If you use mutt from a CW, we will soon (as of cal-meta 0.65) have the default mutt configuration set up to allow a simple keypress to handle spam. Press S on any message to move it to your missed-spam folder. If you are reviewing your spam quarantine, you can just press Y on a non-spam message to put a copy of it in not-spam and also move it back out into your INBOX to deal with properly.

mutt with imapsync

If you use mutt with some form of external IMAP synchronization (such as imapsync), the following keyboard macro (in ~/.mutt/muttrc or ~/.muttrc) can be used to move a missed spam (with full headers) into your local "missed-spam" folder, 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 local maildir folders. It has not been tested with mbox.