Automatic purging of old email in all mailboxes

[Scott W. Sander]

I am using a postfix + Dovecot server as a test mail server for which some
applications in our test environment use as a target to deliver email so
that our real endusers don't receive messages from our test servers.

A few of the mailboxes in Dovecot receive hundreds of emails per day.  I'd
like to automatically remove all emails in all mailboxes and mailbox
folders that were received more than 90 days prior to the received date.
As I'm a novice Dovecot administrator, I'm not exactly sure what the best
way to accomplish this is, but I've started looking at the "doveadm
expunge" command.  I figure I could create a cron job that calls this
command with the -A switch and that has a search query that finds all
emails older than 90 days.

I'm aware that I can test my query by using the "doveadm search" command.
The problem is that when I do any sort of search query with that command
(e.g. "doveadm search -A NEW"), I receive the following error messages:

Error: User listing returned failure
doveadm: Error: Failed to iterate through some users

I've searched for help with this error, but most of the guidance I'm seeing
refers to making sure that dovecot-sql is configured correctly; however,
I'm using "passwd-file" for the passdb and not a true database.

Here is doveconf -n:

-------

# 2.2.22 (fe789d2): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.13 (7b14904)
# OS: Linux 4.4.0-34-generic x86_64 Ubuntu 16.04.1 LTS ext4
auth_mechanisms = plain login
hostname = mail.domain.test
info_log_path = /var/log/dovecot.log
log_path = /var/log/dovecot.log
mail_location = maildir:/var/mail/vhosts/%d/%n
namespace inbox {
  inbox = yes
  location =
  mailbox "Deleted Items" {
    special_use = \Trash
  }
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox "Junk E-Mail" {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Items" {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
}
passdb {
  args = scheme=PLAIN username_format=%u /etc/dovecot/dovecot-users
  driver = passwd-file
}
protocols = " imap lmtp pop3"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
service imap-login {
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
service pop3-login {
  inet_listener pop3s {
    port = 995
    ssl = yes
  }
}
ssl = required
ssl_cert = </etc/ssl/certs/appserver4_combined.pem
ssl_key = </etc/ssl/private/appserver4_key.pem
userdb {
  args = uid=vmail gid=vmail home=/var/mail/vhosts/%d/%n
  driver = static
}

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Thank you in advance!