Backing up an IMAP mailbox yourself: imapsync, Thunderbird, PST export: what people often miss
For one or two mailboxes, free or cheap tools will do. You just need to know what each one really copies, and what it leaves out.
On paper, backing up a mailbox is simple: copy the messages somewhere other than your provider on a regular basis, and keep the ones that get deleted. Plenty of tools can do it, from email apps to command-line utilities.
This article goes through the four most common methods, then the things home-made backups most often miss.
1. Thunderbird and its export add-on
Thunderbird downloads a copy of your messages to the computer. That copy is not a backup: Thunderbird syncs with the server over IMAP, so a message deleted on the server disappears from the computer too.
The ImportExportTools NG add-on can export folders as EML, HTML, PDF or plain text files, and back up the Thunderbird profile automatically when the app closes. It is free and fairly easy to set up on one computer.
The catch: it depends on a computer that is switched on and an app that gets opened regularly, the backup stays on that same computer unless someone copies it elsewhere, and a profile backup taken after a deletion no longer holds the message.
2. Outlook’s PST export
In classic Outlook for Windows, File > Open & Export > Import/Export creates a .pst file holding an account’s messages. The new Outlook for Windows also has an export, under Settings > Files > Export.
It is a snapshot. You make it by hand, and an export from January holds nothing that arrived afterwards. Large .pst files can also get damaged, which is why Microsoft ships a repair tool for them.
3. imapsync, to copy to another mailbox
imapsync copies one IMAP mailbox to another, on the same server or a different one. It is the standard tool for migrations, and it works for backups too: you copy your working mailbox to a spare one with another provider.
By default, imapsync deletes nothing in the destination mailbox, so a message removed from the source stays in the copy. The --delete2 option does the opposite, and you should leave it off for a backup.
The source code is free to use. Ready-made builds for Windows and Mac, and support, are paid: 72 € for the software for professional use, according to the author’s site. You also need a destination mailbox, a computer or server to run the copies, and a scheduled task to repeat them.
4. mbsync and imap-backup, on the command line
mbsync, from the isync project, copies an IMAP mailbox into local folders in Maildir format. By default it syncs everything, deletions included. To use it as a backup, restrict it to pulling new messages from the server, with options such as Sync Pull New and Expunge None.
imap-backup, written in Ruby, is built for backups. In its default mode it keeps a local copy of every message, even after it is deleted from the server, and it can upload messages back to a mailbox.
Both are reliable and free, but they suit someone who can write a config file, schedule a job and read an error log.
What home-made backups tend to miss
The tool matters less than what surrounds it. The same gaps keep coming up:
- failure alerts: a changed password or a server refusing the connection stops the backup, and nobody notices for months;
- location: a copy on the same computer as the email app, or on a drive that is always plugged in, goes with it after a theft, a failure or ransomware;
- the password: it often sits in plain text in a config file, where a revocable app password would be safer;
- disk space: a backup that keeps everything grows every month, until the disk is full;
- a restore test: a backup can turn out to be unusable on the day you need it, unless you have already tried restoring a message.
When it is enough
For a personal mailbox, or two or three mailboxes looked after by someone who knows these tools, a home-made backup works well, as long as the points above are covered.
Beyond that, or when nobody has time to watch scheduled jobs, the real cost is the time spent. That is the case istorel is built for: it does what these tools do, checks every backup and alerts you when one fails. To see what an IMAP backup actually copies, read IMAP backup, explained.