This function causes a fetch of the complete, unfiltered RFC2822 format header of the specified message as a text string and returns that text string.
The options are:
FT_UID - The msgno argument is a UID
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)