This should be fixed in the current SmarterMail 5.5 BETA. The cause of the problem is the zip library we use. We upgraded it for SmarterMail 5.1. Well, part of the upgrade was 64 bit support (for files > 4GB), which apparently, Microsoft's zip handler can not handle. In any case, we turned 64 bit support off for 5.5.
jeffpatrick: It is slightly annoying to have to pay for support when there is clearly a bug causing the problem. I know that has often been repeated here in the forum. :-(
Since it is a bug, the incident will be refunded.
Additionally, please note that these zip files are NOT corrupt. You can still use them with more "advanced" utilities like WinZip or WinRAR or 7Zip.