dovecot - Jun 2015 - privilege of process executed by extprograms plugin

Hi,

I'm just a beginner of extprograms plugin of sieve.
When I execute a process via execute or pipe command,
the process start with "root" privilege.  Why?
I believe the process must be executed with recipient privilege.
Give me some checkpoints, or hints, please.

My setup is as follows:
Ubuntu 14.04.2 LTS 64bit + dovecot-core, dovecot-sieve
plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
  sieve_execute_bin_dir = /some/where
  sieve_extensions = +vnd.dovecot.execute +vnd.dovecot.pipe
  sieve_pipe_bin_dir = /some/where
  sieve_plugins = sieve_extprograms
}
protocol lmtp {
  mail_plugins = " sieve"
  postmaster_address = postmaster at some.where
}

Yakoh