I installed 2.3.4 and just used it with the config files for 2.3.3 without changing anything in the configuration.
I then realized that the LDA was throwing errors.

2018-11-24 00:02:51 1gQIaw-000AZS-Bc </var/spool/virtual/crownkenya.com/john.doe/Maildir>: dovecot_virtual_delivery transport output: lda(john.doe@our.domain.name)Error: net_connect_unix(/var/run/dovecot//stats-writer) failed: Permission denied

I checked on the presence of the sockets in /var/run/dovecot:

srw------- 1 root wheel 0 Nov 24 09:07 stats-reader
srw-rw---- 1 root dovecot 0 Nov 24 09:07 stats-writer

What user/group does dovecot_virtual_delivery run as? Change the stats-writer socket's owner to be that user. For example:

service stats {

unix_listener stats-writer {

user = vmail

}

Or alternatively change dovecot_virtual_delivery to use dovecot group.

I have tried to find any mention of stats-{writer|reader} in the example configs shipped with 2.3.4 and found nothing. I have backed-off 2.3.4 for now till I can figure out how to assign proper permissions to these sockets - or just to figure out why by default, permission is being denied.

Looks like this is happening now because in earlier versions the dovecot-lda process wasn't sending any statistics.