The DISCUSSION_ADD_HEADERS variable allows you to add custom headers to the emails posted in the forum.
You can choose to add or remove headers by specifying the particular information you want to be added or dropped from the header. For example, if you add <#d#> in the Add header field, the URL of that discussion will be added to the header of all the available messages in that discussion.
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Add one or more header names. The match of any of these headers in an outgoing message (via email) causes its addition with appropriate notification to the posting user.