libmail-thread-perl 2.55-1 (arm64 binary) in ubuntu trusty

 The Mail::Thread module implements something relatively close to
 Jamie Zawinski's mail threading algorithm, as described by
 http://www.jwz.org/doc/threading.html. This algorithm is based
 on following References and In-Reply-To headers, and is able to
 deal with not having all of the messages in a thread.