It's apparent I have some level of corruption in my mailboxes - something to do with the combination of mdbox, sis, and zlib. I should note that I know I have a mix of compressed and raw messages - if that makes any difference in identifying the problems.
I think I need to somehow regenerate the mailboxes with issues. Is there a suggestion for how to accomplish this? Should I perhaps use dsync to shift to a temporary mail location and then rename it back? If I DO use dsync in this manner, and I have zlib enabled - will this then compress all the messages in the new mail storage? -- Daniel