Debian dovecot 2.4.1 /etc/dovecot/conf.d/20-lmtp.conf has the lines "mail_plugin" and "sieve = yes" commented-out by default.
dovecot.conf should have a line at the bottom "!include_try local.conf". Uncomment this line. Place the above lines in /etc/dovecot/local.conf.
Always run doveconf for syntax correctness. Invalid syntax in dovecot.conf will log.
FYI: Many dovecot sections and variables can be overridden by later occurrences, but some are not. I run dovecot on Debian. Debian dovecot.conf concatenates conf.d/* into one file. The "passwd pam" { ... } cannot be overridden. Always run doveconf to see how many passwd and userdb sections show-up.
Pro-tip: Remove the include conf.d/* lines in dovecot.conf. and just manually concatenate the conf.d/* lines in dovecot.conf. Everything in one place and your configuration won't break when an upgrade occurs. Do you want to have many changes in multiple files under conf.d/*?
I filed a security bug with dovecot 2.4.1. I have not heard back. I read that I am supposed to file this kind of bug with the Linux Distro, Debian. The bug is 2.4.1 will give the same uid/gid to any account after the first authentication. Everyone else gets the same uid/gid and all clients start downloading the folders/email of this first uid/gid.
2.4.1 is not production ready.
Stay with 2.3.+ until it is ready.