One of Exchange's functions has been to provide connectivity with legacy or third-party mail solutions, including...
Microsoft's own MSMAIL product, which was supplied with earlier versions of Windows. MSMAIL was strictly designed as an interoffice and intra-office mail system, with no inherent Internet/POP/SMTP capabilities, and is no longer supported or manufactured.
When Exchange was first introduced, some degree of backwards compatibility had to be built into the product. To that end, Microsoft created the Shadow Post Office component, an interface to connect MSMAIL gateways to Exchange and allow existing MSMAIL operations to be subsumed into Exchange. Since it comes with Exchange and has a somewhat generic-sounding name, many administrators install the MSMAIL Shadow Post Office without thinking. Unless there is an actual MSMAIL circuit being fed into it, this is unneeded, and may be harmful.
One of the peculiar symptoms of having a Shadow Post Office set up with no MSMAIL circuit feeding into it is an error message: "There is not enough disk space on the Shadow Post Office for the MTA to continue transferring mail. The MTA requires at least 1MB of free disk space on the disk containing the Shadow Post Office files in order to transfer mail. As soon as this space is made available the MTA will continue running." This error may present itself even though there is plenty of free space available. In such a case, if there is no actual MSMAIL circuit, remove the Shadow Post Office by running Exchange Setup and uninstalling the MS Mail Connector. (The cc:Mail Connector is sometimes also installed by mistake, and can be similarly problematic if there is no cc:Mail circuit in the organization.)