I tried SIS a couple years ago - I was very excited with the resulting decrease in storage requirements but the undiagnosed intermittent issues became too significant to ignore so I switched away.  Recently I was thinking about it again.

The primary issue with SIS seemed to be links would be deleted even though the source attachment files and related mails still existed.  It was possible to either manually re-build the links or have a script scan the mail error log and perform such.

I haven't looked at the code - but a thought for a possible "temporary" fix:

    1.  Whatever function in dbox code that performs the deletion of links - prior to actually deleting call a new function that will verify if any mails exist that reference it.  A new function, without modifying existing code, may catch something the existing functions don't - and if it logs the fact that it was called and found something...perhaps we can find the flaw in the original algorithm.  Just a thought.

    2.  In the mail retrieval function, if the attachment link doesn't exist - perform the relevant scan through the attachment database and if found re-create the link automatically.  This should log an error but indicate the recovery.

-- Daniel