C / C++ / MFC

[Serializable]
public __gc class MSGDATA
{
public:
    ...
    MemoryStream * attachStream __gc[];
};

MSGDATA pMsgData = new MSGDATA;
pMsgData.attachStream = new MemoryStream[HOW_MANY_ATTACHMENTS];
for (int k=0; k < HOW_MANY_ATTACHMENTS; k++)
{					
    pMsgData.attachStream[k] = new MemoryStream();
    // read data to stream
}
SendMail(pMsgData);
// flush streams

#define MAX_FILES 20 //Set to what you need.

struct StreamInfo
{
    int nNumberOfFiles;
    DWORD dwFileSizes[MAX_FILES];
};