I am going to use 'dspam' from within a milter where a message is often checked for multiple recipients. Currently, every recipient will get another signature and message must be cloned for every recipient to add the corresponding signature. Things are even worse when two recipients are resolved to the same user by external lookup. Then, message is processed twice by 'dspam' but user is told only one signature.
Hence, it would be nice when a message creates always the same signature. Returning a SHA1 based HMAC code (see RFC 2104) of the message and a configurable secret key would be perfect.