This is certainly interesting. In the grand problem of making sense of bounce messages, there seems to be three sub-problems: 1) Determining if the message is

This is certainly interesting. In the grand problem of making sense of bounce messages, there seems to be three sub-problems: 1) Determining if the message is in fact a bounce. VERP doesn't seem to solve this. 2) Correctly linking a bounce message to the original message. VERP seems to solve that via encoding the message ID into the return path. I could implement VERP with "plus addressing". Thanks! 3) Identifying the error that the bounce message represents. Unfortunately, the presence of HCMSSCs in the hash format does not seem common. Even Gmail's bounce messages do not provide them.