Dovecot and zlib: Mails arrive compressed

[Markus Grunwald <markus@the-grue.de>]

Hello,

to save some precious space on my server, I wanted to enable zlib
compression in dovecot. I followed these instructions:

https://wiki2.dovecot.org/Plugins/Zlib

The relevant parts of my dovecot.conf

protocol imap {
 # Support for dynamically loadable plugins. mail_plugins is a space
separated
  # list of plugins to load.
  mail_plugins = $mail_plugins imap_zlib
...
}

plugin {
  # Used by both the Sieve plugin and the ManageSieve protocol
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
  zlib_save_level = 6 # 1...9
  zlib_save = gz # gz or bz2
}

protocol lda {
  # Support for dynamically loadable plugins. mail_plugins is a space
separated
  # list of plugins to load.
  mail_plugins = $mail_plugins sieve zlib
  mail_plugin_dir = /usr/lib/dovecot/modules
  ...
}

doveconf -n showed no error, I restarted dovecot.
Now when I receive a new mail, it is compressed - but it stays
compressed :( Meaning: It arrives in Thunderbird, but it's not readable.
If I look at the mail sources in Thunderbird, it looks like zlib
compressed text...

What have I missed?

-- 
Markus Grunwald
https://www.the-grue.de/~markus/markus_grunwald.gpg