Conversion script header access
When performing conversions on a message part, the conversion channel has general read access (and modification access limited to specific MIME header lines) to the headers in an enclosing part, an enclosing message/rfc822 part, or to the outermost message headers if there is no enclosing message/rfc822 part. (For more general modifications of message header lines, beyond the attachment labelling modifications available to the conversion channel, see instead the Sieve editheader extension.)
For instance, the IN-A1-TYPE and IN-A1-FORMAT parameters can be used to check the A1-Type: and A1-Format: headers of an enclosing part, and the OUT-A1-TYPE and OUT-A1-FORMAT parameters can be used to set those enclosing headers. Or decisions about interior message part processing can be made based upon the message's outermost headers.
More generally, if an entry is selected that has
ORIGINAL-HEADER-FILE=1, then the headers of that part are written to the file represented by the
INPUT_HEADERS symbol. If an entry is selected that has
MESSAGE-HEADER-FILE=1, then all the original headers of the enclosing message/rfc822 part are written to the file represented by the
MESSAGE_HEADERS symbol. Or if an entry is selected that has
MESSAGE-HEADER-FILE=2, then all the original headers of the outermost message are written to the file represented by the
OVERRIDE-HEADER-FILE=1, then the conversion channel will read and use as the MIME headers on that enclosing part the contents of the file represented by the
OUTPUT_HEADERS symbol. Currently, the supported MIME headers that may be set in this fashion include Content-type:, Content-disposition:, Content-transfer-encoding:, Content-mode:, Content-id:, Content-description:, Content-language:, Content-annotation:, and Content-comments:.