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Just a guess, but I expect the difference is because NetApp de-dupes by checksumming blocks and mark whole blocks as duplicates if they have the same checksum.

True, the start of the message is always at byte 0, but because of different header length per user for the same message (different mail address with different lengths) the body will never start at the same byte.

Oh yes, that's right. I confused maildir format with sendmail queue files that separates header and body. There is still some similarlity for mass mailouts to the same mail domain: they will have almost identical headers +/- message IDs and a few bytes here and there, but as you say, SiS is the way to go for deduping bulky message attachments.

Joseph Tam <jtam.home@gmail.com>