This section describes keywords that deal with header and envelope information. It consists of the following sections:
The contents of header lines are interpreted only when necessary. However, MIME messages can contain multiple sets of message headers as a result of the ability to imbed messages within messages (message/RFC822). The MTA normally only interprets and rewrites the outermost set of message headers. The MTA can optionally be told to apply header rewriting to inner headers within the message as well.
This behavior is controlled by the use of the noinner and inner keywords. The keyword noinner tells the MTA not to rewrite inner message header lines. It is the default. The keyword inner tells the MTA to parse messages and rewrite inner headers. These keywords can be applied to any channel.
The MTA provides per-channel facilities for trimming or removing selected message header lines from messages. This is done through a combination of a channel keyword and an associated header option file or two. Header option file format is described in Header Option Files in Sun Java System Messaging Server 6.3 Administration Reference.
The headertrim keyword instructs the MTA to consult a header option file associated with the channel and to trim the headers on messages queued to that destination channel accordingly, after the original message headers are processed. The noheadertrim keyword bypasses header trimming. The keyword noheadertrim is the default.
The innertrim keyword instructs the MTA to perform header trimming on inner message parts, that is, embedded MESSAGE/RFC822 parts, as well. The noinnertrim keyword, which is the default, tells the MTA not to perform any header trimming on inner message parts.
The headerread keyword instructs the MTA to consult a header option file associated with the channel and to trim the headers on messages enqueued by that source channel accordingly, before the original message headers are processed. Note that headertrim header trimming, on the other hand, is applied after the messages have been processed and is the destination channel, rather than the source channel. The noheaderread keyword bypasses message enqueue header trimming. noheaderread is the default.
Unlike the headeromit and headerbottom keywords, the headertrim and headerread keywords may be applied to any channel whatsoever. Note, however, that stripping away vital header information from messages may cause improper operation of the MTA. Be extremely careful when selecting headers to remove or limit. This facility exists because there are occasional situations where selected header lines must be removed or otherwise limited.
Stripping away header information from messages may cause improper operation of the MTA. Be careful when selecting headers to remove or limit. These keywords are provided for the rare situations where selected header lines must be removed or limited. Before trimming or removing any header line, you must understand the usage of that header line and have considered the possible implications of its removal.
Header options files for the headertrim and innertrim keywords have names of the form channel_headers.opt with channel, the name of the channel with which the header option file is associated. Similarly, header options files for the headerread keyword have names of the form channel_read_headers.opt. These files are stored in the MTA configuration directory, instance_root/config/.
The x_env_to and nox_env_to keywords control the generation or suppression of X-Envelope-to header lines on copies of messages queued to a specific channel. On channels that are marked with the single keyword, the x_env_to keyword enables generation of these headers while the nox_env_to removes such headers from enqueued messages. The default is nox_env_to.
Although it will rarely make sense to do so, the x_env_to keyword can now be used without also setting single on a channel.
The original RFC 822 specification called for two-digit years in the date fields in message headers. This was later changed to four digits by RFC 1123. However, some older mail systems cannot accommodate four-digit dates. In addition, some newer mail systems can no longer tolerate two-digit dates.
Systems that cannot handle both formats are in violation of the standards.
The datefour and datetwo keywords control the MTA’s processing of the year field in message header dates. The keyword datefour, the default, instructs the MTA to expand all year fields to four digits. Two- digit dates with a value less than 50 have 2000 added, while values greater than 50 have 1900 added.
The keyword datetwo instructs the MTA to remove the leading two digits from four-digit dates. This is intended to provide compatibility with incompliant mail systems that require two digit dates; it should never be used for any other purpose.
The RFC 822 specification allows for a leading day of the week specification in the date fields in message headers. However, some systems cannot accommodate day of the week information. This makes some systems reluctant to include this information, even though it is quite useful information to have in the headers.
The dayofweek and nodayofweek keywords control the MTA’s processing of day of the week information. The keyword dayofweek, the default, instructs the MTA to retain any day of the week information and to add this information to date and time headers if it is missing.
The keyword nodayofweek instructs the MTA to remove any leading day of the week information from date and time headers. This is intended to provide compatibility with incompliant mail systems that cannot process this information properly; it should never be used for any other purpose.
Some message transfers, notably some sendmail implementations, cannot process long header lines properly. This often leads not just to damaged headers but to erroneous message rejection. Although this is a gross violation of standards, it is nevertheless a common problem.
The MTA provides per-channel facilities to split (break) long header lines into multiple, independent header lines. The maxheaderaddrs keyword controls how many addresses can appear on a single line. The maxheaderchars keyword controls how many characters can appear on a single line. Both keywords require a single integer parameter that specifies the associated limit. By default, no limit is imposed on the length of a header line nor on the number of addresses that can appear.
The headerlabelalign keyword controls the alignment point for message headers enqueued on this channel; it takes an integer-valued argument. The alignment point is the margin where the contents of headers are aligned. For example, sample header lines with an alignment point of 10 might look like this:
To: email@example.com From: firstname.lastname@example.org Subject: Alignment test
The default headerlabelalign is 0, which causes headers not to be aligned. The headerlinelength keyword controls the length of message header lines enqueued on this channel. Lines longer than this are folded in accordance with RFC 822 folding rules.
These keywords only control the format of the headers of the message in the message queue; the actual display of headers is normally controlled by the user agent. In addition, headers are routinely reformatted as they are transferred across the Internet, so these keywords may have no visible effect even when used in conjunction with simple user agents that do not reformat message headers.
Processing of long header lines containing lots of addresses can consume significant system resources. The maxprocchars keyword is used to specify the maximum length header that the MTA can process and rewrite. Messages with headers longer than this are still accepted and delivered; the only difference is that the long header lines are not rewritten in any way. A single integer argument is required. The default is processing headers of any length.
The sensitivity checking keywords set an upper limit on the sensitivity of messages that can be accepted by a channel. The default is sensitivitycompanyconfidential; messages of any sensitivity are allowed through. A message with no Sensitivity: header is considered to be of normal, that is, the lowest, sensitivity. Messages with a higher sensitivity than that specified by such a keyword is rejected when enqueued to the channel with an error message:
message too sensitive for one or more paths used
Note that the MTA does this sort of sensitivity checking at a per-message, not per-recipient, level: if a destination channel for one recipient fails the sensitivity check, then the message bounces for all recipients, not just for those recipients associated with the sensitive channel.
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.”