Pakete: 
  * ''dovecot-lmtpd dovecot-sieve dovecot-managesieved'' (Debian 9)

<file txt /etc/dovecot/conf.d/20-lmtp.conf>

protocol lmtp {
  mail_plugins = $mail_plugins sieve
}
</file>

<file txt /etc/dovecot/conf.d/90-sieve.conf>
…
plugin {
…
  sieve = file:~/sieve
…
</file>

Testen:
  doveconf -f service=lmtp mail_plugins
  
===== Syntax =====

<code sieve>
require "fileinto";

#
# 2019-2-13
#
if header :contains "subject" "Termin" {
    fileinto "Inbox.Termin";
    stop;
}
</code>

Doku: 
  * https://de.wikipedia.org/wiki/Sieve
  * https://pigeonhole.dovecot.org/
  * https://doc.dovecot.org/2.3/configuration_manual/sieve/#sieve