Outlook provides a feature to save messages to a file in a format that preserves most of the attributes of a message and one that can later be imported into a MAPI folder. The MSG file format, as it is known, is actually a Structured Storage Compound document file. MAPI provides a mechanism to wrap the IMessage interface (the MAPI way of accessing a message's data) over the top of Structured Storage; Outlook uses this feature to provide a simple way of exporting data to a file system.
However, the Structured Storage format has several limitations and is not an efficient file type for MAPI data. PowerMAPI provides an alternative to MSG files as PackedBinary files. Such files are non-editable, archival types that can also be imported. In contrast to MSG files, PowerMAPI's Packed Binary format is able to preserve ALL MAPI properties for a message while the MSG format inherently loses many properties that are considered "calculated" like the message size (as reported by MAPI from the original message store) and the modified date of the item. If legal preservation of MAPI items are important, the Packed Binary format is a better option.