These keywords control Message-hash: headers on messages. Generatemessage specified on a destination channel, causes a Message-hash: header field to be inserted into the message. Keepmessagehash will cause any existing Message-hash: field to be retained. Deletemessagehash will delete any existing Message-hash: field. Deletemessagehash is the default.
The value placed in Message-Hash: fields is a hash of the message. Several new MTA options control how the hash is generated:
MESSAGE_HASH_ALGORITHM - The hash algorithm. Can be any of the following: md2, md4, md5 (the default), sha1, md128 (for RIPE-MD128), or md160 (for RIPE-MD160).
MESSAGE_HASH_FIELDS - Comma separated list of fields from the header to hash (in order). Any known header field can be specified. If this option is not specified it defaults to "message-id,from,to,cc,bcc, resent-message-id,resent-from,resent-to,resent-cc,resent-bcc, subject,content-id,content-type,content-description.”