FT_INTERNAL - The return string
is in "internal" format, without any attempt to
canonicalize to CRLF newlines
FT_PREFETCHTEXT - The RFC822.TEXT
should be pre-fetched at the same time. This avoids an
extra RTT on an IMAP connection if a full message text is
desired (e.g. in a "save to local file" operation)
Return Values
Returns the header of the specified message as a text string.