Recently, I had to migrate mail boxes for a couple of users from one mail provider to another. Both mail providers used IMAP, so I looked into IMAP related synchronization methods. I quickly found the imapsync application, also supported through Gentoo's repository.

What I required

The migration required that all mails, except for the spam and trash e-mails, were migrated to another mail server. The migrated mails had to retain their status flags (so unread mails had to remain unread while read mails had to remain read), and the migration had to be done in two waves: one while the primary mail server was still in use (where most of the mails where synchronized) and then, after switching the mail servers (which was done through DNS changes) re-sync to fetch the final ones.

I did not get access to the credentials of all mail boxes, but together with the main administrator we enabled a sort-of shadow authentication system (a temporary OpenLDAP installation) in which the same users were enabled, but with passwords that will be used during the synchronization. The mailservers were then configured to have a secondary interface available which used this OpenLDAP rather than the primary authentication that was being used by the end users.

Using imapsync

Using imapsync is simple. It is a command-line application, and everything configurable is done through command arguments. The basic ones are of course the source and target definitions, as well as the authentication information for both sides.

~$ imapsync \
  --host1 src-host --user1 src-user --password1 src-pw --authmech1 LOGIN --ssl1 \
  --host2 dst-host --user2 dst-user --password2 dst-pw --authmech2 LOGIN --ssl2

The use of the --ssl1 and --ssl2 is not to enable an older or newer version of the SSL/TLS protocol. It just enables the use of SSL/TLS for the source host (--ssl1) and destination host (--ssl2).

This would just start synchronizing messages, but we need to include the necessary directives to skip trash and spam mailboxes for instance. For this, the --exclude parameter can be used:

~$ imapsync ... --exclude "Trash|Spam|Drafts"

It is also possible to transform some mailbox names. For instance, if the source host uses Sent as the mailbox for sent mail, while the target has Sent Items, then the following would enable migrating mails between the right folders:

~$ imapsync ... --folder "Sent" --regextrans2 's/Sent/Sent Items/'

Conclusions and interesting resources

Using the application was a breeze. I do recommend to create a test account on both sides so that you can easily see the available folders, source and target naming conventions as well as test if rerunning the application works flawlessly.

In my case for instance, I had to add --skipsize so that the application does not use the mail sizes for comparing if a mail is already transferred or not, as the target mailserver showed different mail sizes for the same mails. This was luckily often documented on the various online tutorials about imapsync, such as

• Moving to Google Apps with imapsync on seagrief.co.uk
• Guide to imapsync on wiki.zimbra.com

The migration took a while, but without major issues. Within a few hours, the mailboxes of all users where correctly migrated.