обрабатывать сообщения (process messages)
Входные файлы (Input files)
When mailx is used as described by this volume of POSIX.1‐2017,
the file option-argument (see the -f option) and the mbox shall
be text files containing mail messages, formatted as described in
the OUTPUT FILES section. The nature of the system mailbox is
unspecified; it need not be a file.
