+ message_id = Ustrchr(h->text, ':') + 1;
+ while (isspace(*message_id)) message_id++;
+ fprintf(f, "In-Reply-To: %s", message_id);
+ }
+
+/* Generate a References header if there is at least one of Message-ID:,
+References:, or In-Reply-To: (see RFC 2822). */
+
+for (h = header_list; h != NULL; h = h->next)
+ if (h->type != htype_old && strncmpic(US"References:", h->text, 11) == 0)
+ break;
+
+if (h == NULL)
+ for (h = header_list; h != NULL; h = h->next)
+ if (h->type != htype_old && strncmpic(US"In-Reply-To:", h->text, 12) == 0)
+ break;
+
+/* We limit the total length of references. Although there is no fixed
+limit, some systems do not like headers growing beyond recognition.
+Keep the first message ID for the thread root and the last few for
+the position inside the thread, up to a maximum of 12 altogether. */
+
+if (h != NULL || message_id != NULL)
+ {
+ fprintf(f, "References:");
+ if (h != NULL)
+ {
+ uschar *s, *id, *error;
+ uschar *referenced_ids[12];
+ int reference_count = 0;
+ int i;
+
+ s = Ustrchr(h->text, ':') + 1;
+ parse_allow_group = FALSE;
+ while (*s != 0 && (s = parse_message_id(s, &id, &error)) != NULL)
+ {
+ if (reference_count == sizeof(referenced_ids)/sizeof(uschar *))
+ {
+ memmove(referenced_ids + 1, referenced_ids + 2,
+ sizeof(referenced_ids) - 2*sizeof(uschar *));
+ referenced_ids[reference_count - 1] = id;
+ }
+ else referenced_ids[reference_count++] = id;
+ }
+ for (i = 0; i < reference_count; ++i) fprintf(f, " %s", referenced_ids[i]);
+ }
+
+ /* The message id will have a newline on the end of it. */
+
+ if (message_id != NULL) fprintf(f, " %s", message_id);
+ else fprintf(f, "\n");