Compress mail

compress-mail is a simple command line tool that is used with Dovecot to compress Maildir format files.

Table of Contents

• Compress mail
  • Table of Contents
  • Introduction
  • Installation
  • Usage

Introduction

compress-mail is a simple command line tool that is used with Dovecot to compress Maildir format files.

It follows Dovecot best practices, including its own implementation of the maildirlock program, originally written in C and that is buggy. The Dovecot authors suggest using dsync (doveadm sync), but this only does whole mailboxes, not individual folders, and is more time consuming to configure.

compress-mail uses Maildir flags to mark which files have already been compressed, and thus skips them on subsequent runs, so it is safe to run multiple times.

It will also skip anything that doesn't have the S= size flag. The reasoning is from the Dovecot authors, who say:

All mails must have ,S=<size> in their filename where 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.

I found that if I just move those mails to a temp folder and back again in a MUA (like Thunderbird), it will automatically add the ,S=<size> to the filenames.

Installation

The recommended way to install compress-mail is to use pipx.

After getting pipx installed, simply run:

pipx install compress-mail

Please don't use pip system-wide.

You can of course also install it using classic virtualenvs.

Usage

Usage help can be obtained by running compress-mail --help.

If you have a normal Maildir structure like the following:

drwx------ 2 user group  2363392 Mar 31 08:42 cur
-rw------- 1 user group   242824 Mar 30 15:41 dovecot.index
-rw------- 1 user group 15596760 Mar 31 08:18 dovecot.index.cache
-rw------- 1 user group    26224 Mar 31 08:42 dovecot.index.log
-rw------- 1 user group       97 Feb 15  2019 dovecot-keywords
-rw------- 1 user group   953295 Mar 31 08:18 dovecot-uidlist
-rw------- 1 user group        0 Mar  2  2011 maildirfolder
drwx------ 2 user group    36864 Mar 31 08:17 new
drwx------ 2 user group   270336 Mar 31 08:18 tmp

.Trash
├── cur
├── new
└── tmp

All you need to do is run:

user@host:~$ compress-mail --basedir .Trash --compression zstd