dovecot replication and pop3 deletion

[Nigel Metheringham]

Hi,

I am investigating the use of dovecot for a voicemail storage system 
(which is effectively a SMTP/POP3 system), which we want to use 
replication to give us some resilience.

Tests have mostly been on dovecot 2.3.8 (9df20d2db) - as packaged on 
Centos 8 - but I have also used a couple of similar versions on 
different platforms, so this is not a specific build problem.

The platform has replication working, over an SSL TCP link, for delivery.

With the pop3 configuration as default as is possible (although the 
mail_plugins are explicitly included into the protocol sections to 
ensure notify and replication is called for), any pop3 delete operation 
does not cause a notify or replication operation.  The two servers show 
the mailboxes in a different state up until something else prods a 
replication into progress.

I would suggest that a pop3 delete/expunge of a message should be 
flagged as a mailbox change can cause a notify/replication, and that the 
current behaviour is a bug.

There are workarounds to this, and I am considering adding a note to the 
wiki page https://wiki.dovecot.org/Replication to flag this - not sure 
what your wiki editing policy is.

# Workaround 1 - Change pop3 deletion to set a flag

If the setting
     pop3_deleted_flag = '$POP3Deleted'

is added, then rather than the mail being deleted, it is flagged, and 
this change causes a notification and replication to the other server.

However since the mail is not actually deleted, you additionally need 
another (cron?) task to periodically remove flagged mail, for example:-

     doveadm expunge  -A MAILBOX inbox KEYWORD '$POP3Deleted'

# Workaround 2 - Use Lazy Expunge

You can use the lazy_expunge plugin - this has the same effect, and 
automates the removal of the messages after a period of time.

     https://doc.dovecot.org/configuration_manual/lazy_expunge_plugin/

For example, you can set the following changed config fragments:-

     mail_plugins = $mail_plugins notify replication lazy_expunge

     namespace inbox {
         separator = /
         prefix =
         inbox = yes

         # automatic lazy expunge mailbox
         mailbox .EXPUNGED {
             autoexpunge = 3 days
             autoexpunge_max_mails = 1000
         }
     }

     plugin {
         # Move messages to an .EXPUNGED mailbox
         lazy_expunge = .EXPUNGED
     }



     Nigel.

-- 

[ Nigel Metheringham --------------------------- nigel at dotdot.cloud ]
[              Ellipsis Intangible Cloudy Technologies              ]