In-Reply-To:

In-Reply-To:

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Identify previous correspondence

RFC2822

The In-Reply-To: header is used to identify previous correspondence that the current message is in reply to. This header is generated by MUAs, not by sendmail . Prior to RFC2822, the field for this header was arbitrary text with one restriction. If that text included the message identifier, that identifier had to be enclosed in angle brackets ( < and > ) and had to adhere to the format for all message identifiers.

Beginning with RFC2822, the In-Reply-To: header can contain only message identifiers, each surrounded by angle braces, and each separated from the next by a comma.

A typical use of the In-Reply-To: header might look like the following:

 In-Reply-To: <[e-mail]>, <[e-mail]>,        <[e-mail]>   whitespace  

The In-Reply-To: header should never be declared in the configuration file.