What is the recommended procedure to migrate from mbox to maildir on a server already running dovecot? Ideally, this procedure would convert messages in an existing mbox INBOX and in ~/mail mboxes (as defined in my dovecot configuration) to storage in a ~/Maildir. Some of the INBOXs are rather large -- 100's of MB (one of the reasons for migrating).

What I have tried has not worked for me. Below, I describe what I tried, how I tested before I tried, what went wrong, and the current setup (pre-migration).

What I tried:

stop exim4

service dovecot stop

Make dovecot listen only locally, not to external connections.

/etc/dovecot/local.conf

protocol imap { listen = 127.0.0.1:143 ssl_listen = 127.0.0.1:993 }

protocol pop3 { listen = 127.0.0.1:110 ssl_listen = 127.0.0.1:995 }

Modify dovecot configuration for maildir: Watch capitalization!

#in 10-mail.conf:

mail_location = maildir:/home/%u/Maildir

namespace inbox { separator = / }

dovecot -n, to confirm results and perform limited syntax checking.

service dovecot start

for each user, in a bash script:

dsync -v -u "$U" mirror "mbox:/home/$U/mail:INBOX=/var/mail/$U"

(if the conversion worked for all users, then reconfigure exim to use maildir and restart it; reconfigure dovecot to listen to external interfaces and restart.)

How I tested:

Before trying this on the real server, I set up a matching (same versions of Debian, Exim, and Dovecot) VM using mbox, with a couple of accounts. One using POP3, one using IMAP. The IMAP account was accessed from Claws and from Outlook. The test accounts had perhaps a dozen messages on the server. I tried the conversion procedure, and everything I tried worked flawlessly. (Yes, next time I will move some real inbox files from the live server).

What went wrong:

I encountered two types of problems here. The first was several dsync warnings (username changed):

dsync(example): Warning: Mailbox changes caused a desync. You may want to run dsync again: Remote lost mailbox GUID 21bfa11a81f11763ac150000d09efc50 (maybe it was just deleted?)

As I understand it, this can result in some or all messages being downloaded again by clients. My understanding may be wrong. I also don't understand how the mailbox could have changed -- exim4 was stopped, dovecot is only listening on 127.0.0.1, and I am the only user who uses a local mail client on that system, and this was not my account.

The second problem I encountered seems to match Debian Bug report logs - #1011238.

In part of the discussion of that report, the reporter mentions adding -1 to the command, as shown:

dsync -v -u "$U" mirror -1 "mbox:/home/$U/mail:INBOX=/var/mail/$U"

While this seemed to eliminate the dsync panic of #1011238, as far as I can tell, no old messages were actually moved from the old mbox files to the new maildirs. At that point I restored the backup mbox files and undid the changes to dovecot and exim4.

Current setup:

Debian 11.4 Exim 4.94.2-7 dovecot 1:2.3.13+dfsg1-2

The current setup with Dovecot was a successful migration from IMAP-UW in December 2020.

Output from dovecot -n (the list of trusted networks has been removed) # 2.3.13 (89f716dc2): /etc/dovecot/dovecot.conf # Pigeonhole version 0.5.13 (cdd19fe3) # OS: Linux 5.10.0-16-686-pae i686 Debian 11.4 # Hostname: localhost auth_verbose = yes mail_location = mbox:~/mail:INBOX=/var/mail/%u mail_privileged_group = mail namespace inbox { inbox = yes location = mailbox Drafts { special_use = \Drafts } mailbox Junk { special_use = \Junk } mailbox Sent { special_use = \Sent } mailbox "Sent Messages" { special_use = \Sent } mailbox Trash { special_use = \Trash } prefix = } passdb { driver = pam } pop3_uidl_format = %08Xv%08Xu protocols = " imap pop3" ssl_cert = </etc/dovecot/dovecot.pem ssl_client_ca_dir = /etc/ssl/certs ssl_dh = # hidden, use -P to show it ssl_key = # hidden, use -P to show it userdb { driver = passwd }

Thank you to anyone who read this far.

Ken