I was having the same problem with zip, dwg, and pdf attachments. It was very frustrating as I turned indexing off but was still getting the same error. The only thing I have found that works reliably so far is to disable indexing for specific file types as described
here. Every time I would see an email getting stuck in the deferred queue I would add an entry to LDAP to disable indexing for it, restart the server, and it would pass on without problems. After going through this a few times I think I've gotten all of the troublesome file types. I haven't had anything show up in deferred in a week or so.
Hope this helps,
Mike Chapman