I would like to send contents of the current buffer to stdin of external command (like mail).
How do I send a Vim buffer to an external command?
You can use :w !cmd to write the current buffer to the stdin of an external command. From :help :w_c:
:w_c :write_c
:[range]w[rite] [++opt] !{cmd}
Execute {cmd} with [range] lines as standard input
(note the space in front of the '!'). {cmd} is
executed like with ":!{cmd}", any '!' is replaced with
the previous command :!.
A related command is :%!cmd which does the same thing and then replaces the current buffer with the output of the command. So :%!sort would invoke the external sort command to sort the current buffer in place. From :help :range!:
:{range}![!]{filter} [!][arg] :range!
Filter {range} lines through the external program
{filter}. Vim replaces the optional bangs with the
latest given command and appends the optional [arg].
Vim saves the output of the filter command in a
temporary file and then reads the file into the buffer
tempfile. Vim uses the 'shellredir' option to
redirect the filter output to the temporary file.
However, if the 'shelltemp' option is off then pipes
are used when possible (on Unix).
When the 'R' flag is included in 'cpoptions' marks in
the filtered lines are deleted, unless the
:keepmarks command is used. Example:
:keepmarks '<,'>!sort
When the number of lines after filtering is less than
before, marks in the missing lines are deleted anyway.