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: <847.193925.780455@hostA.com>, <1021169802.330@HostB.co.th>,
       <200106020731.BAA20313@HostC.br.ca>
    
   whitespace

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