What does this message mean: ”MAPI-Error: MAPI_E_STORE_FULL”

Your pst file is full, that’s the reason for the error message. With Outlook 2003 the format of the pst file was changed. The size of the old format is limited to 2GB only. 

Please delete old items (incl. “Deleted items”) and than compress your pst file (Start -> Control panel -> Mail -> Data files -> Settings -> Compact now) to join the SYNCING.NET "Outlook Group".