[Dovecot] dsync mbox to maildir migration does not delete, deleted e-mails

Hello,

I am planning to migrate all users from mbox to maildir.

I am trying to do it with minimum downtime. (~100GB data)

All users are currently using POP3.

Dovecot version is 2.2.10 (latest). Command used is: dsync -u username mirror maildir:~/Maildir

Process I plan is: (omitting steps related to sendmail / procmail)

1. keep dovecot running (with mail_location as mbox)

2. dsync for all users (this may take 3-4 hours or more)

3. (downtime starts) stop dovecot and sendmail (to stop new e-mails)

4. block pop3, imap ports on firewall (so users can not connect)

5. start dovecot (still with mbox)

6. dsync again to sync e-mails arrived between step 2 and 3

7. dsync again (just to make sure!)

8. (downtime ends) restart dovecot (with mail_location as maildir)

Now, here is my problem.

Lets say there is user joe, who has 50 NEW e-mails in mbox (INBOX).

Step 2 perfectly syncs his 50 e-mails to 'new' folder of maildir.

Now in the mean time, before step 3, he connected via POP3 and downloaded and deleted 50 e-mails.

Now when we reach step 6 (re-sync), what I expected was dsync will detect that 50 e-mails are deleted and it will delete 50 e-mails from 'new' directory of maildir.

But that is not happening. 50 e-mails are still there.

I fear that these e-mails will be re-downloaded on his Outlook once I switch dovecot to maildir.

This will happen for each and every user which will cause huge mess.

So how to tell, dsync to delete non-existent e-mails which are no more there in mbox (INBOX)?

Thanks in advance,

Regards,

A M