libmail-box-perl 2.118-1 (amd64 binary) in ubuntu xenial

 Mail::Box is the base-class for accessing various types of mail-folder
 organizational structures in a uniform way. The various folder types vary
 on how they store their messages. For example, a folder may store many
 messages in a single file, or store each message in a separate file in a
 directory. Similarly, there may be different techniques for locking the
 folders.