This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

Zlib plugin

Zlib plugin can be used to read compressed mbox, maildir or dbox files. It can be also used to write (via IMAP, LDA and/or LMTP) compressed messages to dbox or Maildir mailboxes. Zlib plugin supports compression using zlib/gzip, bzlib/bzip2, liblzma/xz (v2.2.9+) and liblz4/lz4 (v2.2.11+).


# Enable zlib plugin globally for reading/writing:
mail_plugins = $mail_plugins zlib

# Enable these only if you want compression while saving:
plugin {
  zlib_save_level = 6 # 1..9; default is 6
  zlib_save = gz # or bz2, xz or lz4


Compressed mbox files can be accessed only as read-only. The compression is detected based on the file name, so your compressed mboxes should end with .gz or .bz2 extension. There is no support for compression during saving.


Mails can be stored as compressed. Existing uncompressed mails can't currently be directly compressed (or vice versa). You could, however, use dsync to copy all mails to another location (which saves them compressed) and then replace the original location with the new compressed location. You can do this by treating the operation the same as if you were migrating from one mailbox format to another (see the dsync page examples).


When this plugin is loaded Dovecot can read both compressed and uncompressed files from Maildir. If you've enabled both gzip and bzip2 support you can have files compressed with either one of them in the Maildir. The compression is detected by reading the first few bytes from the file and figuring out if it's a valid gzip or bzip2 header. The file name doesn't matter. This means that an IMAP client could also try to exploit security holes in zlib/bzlib by writing specially crafted mails using IMAP's APPEND command. This is prevented by Dovecot not allowing clients to save mails that are detected as compressed.

All mails must have ,S=<size> in their filename where <size> contains the original uncompressed mail size, otherwise there will be problems with quota calculation as well as other potential random failures. Note that if the filename doesn't contain the ,S=<size> before compression, adding it afterwards changes the base filename and thus the message UID. The safest thing to do is simply to not compress such files.

You should also preserve the file's mtime so INTERNALDATE doesn't change.

If you want to use dsync to convert to a compressed Maildir you may need -o maildir_copy_with_hardlinks=no (this is set to yes by default and will prevent compression).


You'll probably want to use some cronjob to compress old mails. However note that to avoid seeing duplicate mails in rare race conditions you'll have to use the included maildirlock utility. The idea is to:

  1. Find the mails you want to compress in a single maildir.
    • Skip files that don't have ,S=<size> in the filename.

  2. Compress the mails to tmp/

    • Update the compressed files' mtimes to be the same as they were in the original files (e.g. touch command)
  3. Run maildirlock <path> <timeout>. It writes PID to stdout, save it.

    • <path> is path to the directory containing Maildir's dovecot-uidlist (the control directory, if it's separate)

    • <timeout> specifies how long to wait for the lock before failing.

  4. If maildirlock grabbed the lock successfully (exit code 0) you can continue.
  5. For each mail you compressed:
    1. Verify that it still exists where you last saw it.
    2. If it doesn't exist, delete the compressed file. Its flags may have been changed or it may have been expunged. This happens rarely, so just let the next run handle it.
    3. If the file does exist, rename() (mv) the compressed file over the original file.

      • Dovecot can now read the file, but to avoid compressing it again on the next run, you'll probably want to rename it again to include e.g. a "Z" flag in the file name to mark that it was compressed (e.g.,S=3271:2,SZ). Remember that the Maildir specifications require that the flags are sorted by their ASCII value, although Dovecot itself doesn't care about that.

  6. Unlock the maildir by sending a TERM signal to the maildirlock process (killing the PID it wrote to stdout).

Plugins/Zlib (last edited 2016-05-07 23:03:46 by TimoSirainen)