When a client application displays a message , the subject is simply a null-terminated string stored in the lpszSubject member of a MapiMessage structure.
When replying to or forwarding a message, modify the subject line. The subject of a reply to a message should begin with the string "RE:", or an appropriate localized equivalent. Similarly, forwarded messages should have subject lines that begin with "FW:" or an appropriate localized equivalent. If the subject line already begins with "RE:" or "FW:", do not add it again.