I recently migrated email providers (several times, actually) and came across the fantastic tool mbsync. It’s included in most GNU/Linux distributions, though sometimes called by its old name, “isync”.
mbsync is very powerful, it can not only sync between a remote IMAP server and e.g. a local on-disk mirror that you want to keep so you can back it up, it can also sync between two remote IMAP servers directly.
Many very advanced configurations are possible from your own personal .mbsyncrc file. But since it took some time to puzzle together a good config from the semi-cryptic manpage, here are two configs I used for two typical situations.
Migrating from one IMAP server to another
IMAPStore oldserver Host mail.myoldserver.com User myusername Pass mypassword IMAPStore newserver Host mail.newserver.org User myusername Pass mypassword Channel migrate Master :oldserver: Slave :newserver: Patterns * !Trash Create Slave Expunge Slave Sync Pull
Calling mbsync migrate will migrate your email from your old server to the new one, creating any new directories and messages on the new server and not deleting anything on the old server. You can run this multiple times in case new messages are still arriving on your old server.
Mirroring your IMAP mail to a local directory
This is useful for backing up your email. Particularly nice if you’re on a free or cheap email provider that does not offer backups.
IMAPStore yourserver Host mail.yourserver.io User yourusername Pass yourpassword MaildirStore mirror AltMap yes Path ~/mailmirror/ Channel mirror Master :yourserver: Slave :mirror: Patterns * !Trash Create Slave Expunge None Sync Pull
Then run mbsync mirror to pull your email. You can run this multiple times, it will not delete messages in the mirror directory. If you want messages to be deleted there, set Expunge to slave. Make sure to back up both the mailmirror directory and the Maildir directory, which holds your INBOX.
A note about AltMap: This particular option was required so that I could mirror to remotely mounted NFS shares. It will make mbsync create its UID validity data in a database instead of in a .uidvalidity file. For whatever reason, it seems that writing the .uidvalidity file stalls forever on NFS shares. If you’re not using an NFS share, you can try without that option.
In both examples, the Trash directory is ignored.