- my $irt = $hdr->header_raw('In-Reply-To');
-
- return mid_clean($irt) if defined $irt && $irt ne '';
-
- my $refs = $hdr->header_raw('References');
- if ($refs && $refs =~ /<([^>]+)>\s*\z/s) {
- return $1;
+ my %mid = map { $_ => 1 } $hdr->header_raw('Message-ID');
+ my @refs = ($hdr->header_raw('References'),
+ $hdr->header_raw('In-Reply-To'));
+ @refs = ((join(' ', @refs)) =~ /<([^>]+)>/g);
+ while (defined(my $irt = pop @refs)) {
+ next if $mid{"<$irt>"};
+ return $irt;