root@mail:/etc/dovecot/conf.d# cat  auth-system.conf.ext
# Authentication for system users. Included from 10-auth.conf.
#
# <doc/wiki/PasswordDatabase.txt>
# <doc/wiki/UserDatabase.txt>

# PAM authentication. Preferred nowadays by most systems.
# PAM is typically used with either userdb passwd or userdb static.
# REMEMBER: You'll need /etc/pam.d/dovecot file created for PAM
# authentication to actually work. <doc/wiki/PasswordDatabase.PAM.txt>
passdb {
  driver = pam
  # [session=yes] [setcred=yes] [failure_show_msg=yes] [max_requests=<n>]
  # [cache_key=<key>] [<service name>]
  args = session=yes cache_key=%s%u dovecot
}

# System users (NSS, /etc/passwd, or similiar).
# In many systems nowadays this uses Name Service Switch, which is
# configured in /etc/nsswitch.conf. <doc/wiki/AuthDatabase.Passwd.txt>
#passdb {
  #driver = passwd
  # [blocking=no]
  #args =
#}

# Shadow passwords for system users (NSS, /etc/shadow or similiar).
# Deprecated by PAM nowadays.
# <doc/wiki/PasswordDatabase.Shadow.txt>
#passdb {
  #driver = shadow
  # [blocking=no]
  #args =
#}

# PAM-like authentication for OpenBSD.
# <doc/wiki/PasswordDatabase.BSDAuth.txt>
#passdb {
  #driver = bsdauth
  # [blocking=no] [cache_key=<key>]
  #args =
#}

##
## User databases
##

# System users (NSS, /etc/passwd, or similiar). In many systems nowadays this
# uses Name Service Switch, which is configured in /etc/nsswitch.conf.
userdb {
  # <doc/wiki/AuthDatabase.Passwd.txt>
  driver = passwd
  # [blocking=no]
  #args = blocking=yes

  # Override fields from passwd
  #override_fields = home=/home/virtual/%u
}

# Static settings generated from template <doc/wiki/UserDatabase.Static.txt>
#userdb {
  #driver = static
  # Can return anything a userdb could normally return. For example:
  #
  #  args = uid=500 gid=500 home=/var/mail/%u
  #
  # LDA and LMTP needs to look up users only from the userdb. This of course
  # doesn't work with static userdb because there is no list of users.
  # Normally static userdb handles this by doing a passdb lookup. This works
  # with most passdbs, with PAM being the most notable exception. If you do
  # the user verification another way, you can add allow_all_users=yes to
  # the args in which case the passdb lookup is skipped.
  #
  #args =
#}