The S.* files in the SIMS queue are plain text files consisting of the message and its headers prefaced with lines containing the SMTP envelope and queue state. I don't know that Stalker ever fully documented the semantics, and I don't remember everything I used to know about SIMS, but I THINK that you can change a terminally failed message back into a retryable message by editing the envelope recipient line(s) at the top starting with 'R E ' to start with 'R W '