Offline Mail in Linux

This page describes my offline mail setup, which is based heavily on Joey Hess' setup, as he described it to me. (Well, that was true when I first wrote this doc, before I moved away from isync).

Intro

IMAP4 is capable of working in "disconnected" mode. This allows you to manipulate messages in your local mailbox while you are offline, and have your changes synced up with the server upon reconnect. In fact, you can keep multiple offline clients in sync in this manner.

Tools

I use the following packages (from the Debian/GNU Linux distribution) for this setup:

• courier-imap-ssl - imap server
• offlineimap - imap client for use in disconnected mode
• postfix - mail transport agent
• mutt - mail user agent
• procmail - mail filtering

Aron Griffis uses a modified version of this approach, documented here.

Overview

I use courier-imap as my imaps server. Authentication occurs over ssl, so no plain text passwords are sent. Each of my clients periodically runs offlineimap (either by hand, or via a cronjob) in order to communicate changes back to the server and receive any new messages.

Recent versions of courier support an "Outbox" enhancement. When a client uploads a message to the Outbox folder on the server, courier can be configured to hand that message off to your MTA. This allows you to compose messages offline, and have them transmitted from your imap server when you reconnect.

The Details

• On the server, apt-get install courier-imap-ssl
• You may need to generate a certificate for your server - to do so, edit /etc/courier/imapd.cnf then run /usr/lib/courier/mkimapdcert. This should generate the file /etc/courier/imapd.pem. Edit /etc/courier/imapd to turn on the Outbox feature, if you want to use it.
• On your clients, apt-get install offlineimap. offlineimap ships with an example config file with verbose comments, and setting it up is pretty self explanatory.
• Maildir style mailboxes must be used. I have a ~/Maildir folder in my home directory on the server that contains various subfolders, the names of which all begin with a "." (this appears to be a requirement for courier-imap). All of my mail is delivered into one of these folders via procmail. Various scripts are available on the net for converting from one mailbox type to another.
• I use procmail to deliver all my messages into Maildir folders in my home directory, instead of postfix's default of adding them to an mbox style folder in /var/mail. Here's the pieces of my procmailrc that are sufficient for copying all new mail into a default Maildir folder:

  	
  MAILDIR=$HOME/Maildir
  DEFAULT=.mbox/
  

  mbox is a poor name of course, I just use it for historical reasons.

Configuring Mutt

Mutt is very configurable, and it is quite easy to integrate into this setup.

• Since I use the Outbox feature, I need to change mutt's default method for sending mail. I add the following line to my muttrc:
  set sendmail="/home/dannf/bin/mailout"
  
  /home/dannf/bin/mailout is a wrapper around the safecat utility, which takes advantage of the maildir structure to prevent race conditions.
  
  mailout:

  	
  #!/bin/sh
  echo "Saving to Outbox..."
  safecat /home/dannf/Mail/Outbox/tmp /home/dannf/Mail/Outbox/new
  

• To sync my folders from within mutt, I add the following macros:
  

  	
  macro index $ "!/usr/bin/offlineimap -a Home\n"
  macro pager $ "!/usr/bin/offlineimap -a Home\n"
  

Multiple Accounts

I like to be able to read both my personal mail and my work mail on my laptop. However, I like to keep the two separate as much as possible. Here's what I do to maintain this separation:

• I don't want my work/personal messages intermingled in the same folders, although I use the same folder names in both places. To keep them distinct, I add a special suffix to the name of my work folders on the client side (e.g., .work). This is easy to setup using the nametrans option in the offlineimaprc file. For example, in my [home] section I have:

  	
  nametrans = lambda foldername: re.sub('^INBOX.', '', foldername)
  

  which just strips off the 'INBOX.' prefix. And in my [work] section, I have:

  nametrans = lambda foldername: re.sub('^INBOX.', '', re.sub('$', '.work', foldername))
  

  which also adds the .work suffix to each folder. I could've just kept them in separate directories, but I use the following muttrc option to dynamically specify my mail folders, and was too lazy to change it:

  mailboxes `find ~/Mail -type d -maxdepth 1 -mindepth 1 -printf "%f "`
  

• I want personal outgoing mail to be sent via my home server, while work mail should only be sent through work servers. To make this work, I have a separate mailout script for sending work e-mail, and I use the following lines in my muttrc to tell mutt when to use it:

  # from Omnic
  set sendmail="/home/dannf/bin/mailout"
  ## use a different Outbox for work mail
  folder-hook ".*\.work" "set sendmail=/home/dannf/bin/mailout.work"
  

  The distinction is made based on the folder I am in when I compose/send out a message. I do similar things to keep my Trash and Sent folders separate. mailout.work copies the message into my Outbox.work folder, which will later sync to my mail server at work.
• I also add the following to my muttrc to keep my From header correct:

  folder-hook ".*\.work" 'my_hdr From: dann frazier <dannf@work.com>'
  

• Because I work behind a firewall, I can't always sync my personal and work accounts at the same time. So, I pass different options to offlineimap, depending on which folder I'm in:

  	
  macro index $ "!/usr/bin/offlineimap -a home\n"
  macro pager $ "!/usr/bin/offlineimap -a home\n"
  folder-hook ".*\.work" 'macro index $ "!/usr/bin/offlineimap -a work\n"
  folder-hook ".*\.work" 'macro pager $ "!/usr/bin/offlineimap -a work\n