Headertrim, noheadertrim, headerkeeporder, headerread, noheaderread, innertrim, noinnertrim Channel Options

Trimming message header lines
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 option and an associated header option file or two. The  channel option 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  option bypasses header trimming, but does cause the MTA to re-order header lines according to its internal header line order defaults (plus any configured ordering rules). New in MS 6.3 is the  channel option, which (making use of new MTA header line handling) preserves whatever header line ordering was present in the message as enqueued. was the default in MS 6.2 and earlier; as of MS 6.3,  is the default.

The  channel option instructs the MTA to perform header trimming on inner message parts, i.e., embedded MESSAGE/RFC822 parts, as well. Note that setting  overrides , causing at a minimum   effect (that is, potential re-ordering of header lines). The  channel option, which is the default, tells the MTA not to perform any header trimming on inner message parts.

The  channel option 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  header trimming, on the other hand, is applied after the messages have been processed, and is destination channel, rather than source channel, related. The  channel option bypasses message enqueue header trimming. is the default.

Unlike the   and   options, the   and   options may be applied to any channel whatsoever. Note, however, that stripping away vital header information from messages may cause improper MTA operation. Be extremely careful when selecting headers to remove or limit. This facility exists because there are occassional situations where selected header lines must be removed or otherwise limited. ''Do not merely trim header lines away because you or your users find them annoying --- those header lines are there for a reason. More often than not, the header lines that users feel are superfluous are among the most important. Before trimming or removing any header line, be sure that you understand the usage of that header line and have considered the possible implications of its removal.''

Header options files for the  and   channel options have names of the form   with   the name of the channel with which the header option file is associated. Similarly, header options files for the  channel option have names of the form. See Header option files for information on the format of these files.

Note that as of MS 6.3, the MTA supports the Sieve  extension, which offers quite a different way to modify message header lines.

See also:
 * headercut Option
 * Header option files
 * Sieve filters
 * headerbottom Option
 * Headers channel options
 * Channel options