Dovecot logins fail for virtual users (permission denied, we're not in group 5000)

Question

I have:

• Ubuntu 17.04
• Dovecot 2.2.27 (c0f36b0)
• Postfix 3.1.4
• regular unix users with mail in /home/bob/Maildir
• virtual users with mail in /var/mail/vhosts/domain.com/bob/Maildir

Regular users can receive mail as expected. When virtual users try to log in, it fails. The log shows an error like this:

2017-09-14 22:44:55 auth: Error:
passwd-file(travler at travler.net,54.210.254.169,<EYwzYDFZWoY20v6p>):
stat(/var/mail/vhosts/travler.net/shadow) failed: Permission denied
(euid=114(dovecot) egid=121(dovecot) missing +x perm: /var/mail/vhosts,
we're not in group 5000(vmail), dir owned by 5000:5000 mode=0750)

A previous user had a similar problem in 2015: https://dovecot.org/list/dovecot/2015-July/101469.html

He was advised to look at https://wiki2.dovecot.org/UserIds … without being given more specific advice. I looked at the page. I tried adding mail_uid and mail_gid. No luck.

In my case, I see:

# sudo -u dovecot groups
dovecot vmail

# sudo -u dovecot cat /var/mail/vhosts/travler.net/shadow
<file contents>

I have a vmail user:

# id vmail
uid=5000(vmail) gid=5000(vmail) groups=5000(vmail)

# ls -la /var/mail/vhosts
total 0
drwxr-s--- 1 vmail vmail 246 Jun 30 11:23 .
drwxrwsrwt 1 mail  mail   26 Dec  5  2016 ..
drwxr-s--- 1 vmail vmail  66 Sep 14 23:11 travler.net

# ls -l /var/mail/vhosts/travler.net/
total 8
-rw-r----- 1 vmail vmail 174 Dec  5  2016 passwd
-rw-r----- 1 vmail vmail 173 Dec  5  2016 shadow
drwxr-s--- 1 vmail vmail 450 Sep  9 13:16 travis.belmont
drwxr-s--- 1 vmail vmail 750 Sep 13 14:14 travler

I've set mail_gid and mail_uid

# doveconf  -n|grep id
mail_gid = 5000
mail_uid = 5000

Password file has lines this this:

travler::5000:5000::/var/mail/vhosts/travler.net/travler::userdb_mail=maildir:~

Here's my full config:

# doveconf -n
# 2.2.27 (c0f36b0): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.16 (fed8554)
# OS: Linux 4.10.0-33-generic x86_64 Ubuntu 17.04 btrfs
log_path = /var/log/dovecot
log_timestamp = "%Y-%m-%d %H:%M:%S "
mail_gid = 5000
mail_location = maildir:/home/%u/Maildir
mail_privileged_group = mail
mail_uid = 5000
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
}
passdb {
  args = username_format=%n /var/mail/vhosts/%d/shadow
  driver = passwd-file
}
plugin {
  sieve = file:~/sieve;active=~/.dovecot.sieve
}
protocols = " imap"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service imap-login {
  inet_listener imap {
    port = 0
  }
}
ssl_cert = [snip]
ssl_cipher_list = [snip]
ssl_dh_parameters_length = 2048
ssl_key =  # hidden, use -P to show it
ssl_protocols = !SSLv3 !SSLv2
userdb {
  driver = passwd
}
userdb {
  args = username_format=%n /var/mail/vhosts/%d/passwd
  driver = passwd-file
}

What do I need to set to get dovecot to run as vmail / fix this error?