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