обрабатывать сообщения (process messages)
Стандартный ввод (Stdin)
When mailx is invoked in Send Mode (the first synopsis line),
standard input shall be the message to be delivered to the
specified addresses. When in Receive Mode, user commands shall
be accepted from stdin. If the User Portability Utilities option
is not supported, standard input lines beginning with a <tilde>
('~') character produce unspecified results.
If the User Portability Utilities option is supported, then in
both Send and Receive Modes, standard input lines beginning with
the escape character (usually <tilde> ('~')) shall affect
processing as described in Command Escapes in mailx.
