Наверное так будет проще: выложу конф файлы, а Вы уж пинайте меня за ошибки...

### DOVECOT CONF ###

base_dir = /var/run/dovecot/
protocols = imap pop3
listen = *

disable_plaintext_auth = no
shutdown_clients = yes
ssl_disable = yes

log_path = /var/log/dovecot.log
info_log_path = /var/log/dovecot-info.log
log_timestamp = "%b %d %H:%M:%S "
syslog_facility = mail

login_dir = /var/run/dovecot/login
login_user = dovecot

login_process_size = 64
login_process_per_connection = yes
login_processes_count = 3
login_max_processes_count = 64
login_max_connections = 100

login_greeting = I am ready, sir.

login_log_format_elements = user=<%u> method=%m rip=%r lip=%l %c
login_log_format = %$: %s

mail_location = maildir:/var/mail/%d/%n

first_valid_uid = 25
first_valid_gid = 0
umask = 0077

protocol imap {
}

protocol pop3 {
pop3_uidl_format = %08Xu%08Xv
}

protocol lda {
  postmaster_address = admin@****
  auth_socket_path = /var/run/dovecot/auth-master
}

auth default {
mechanisms = plain
   socket listen {
     master {
       path = /var/run/dovecot/auth-master
       mode = 0600
       user = mailnull
     }
   }

passdb passwd-file {
    args = /etc/dovecot.passwd
   }

userdb static {
       args = uid=vmail gid=vmail home=/var/mail/%d/%n
}

user = root
}

plugin {

}

Фрагмент от
####EXIM####

begin routers

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more

system_aliases:
  driver = redirect
  allow_fail
  allow_defer
  data = ${lookup{$local_part}lsearch{/etc/aliases}}
  file_transport = address_file
  pipe_transport = address_pipe

userforward:
  driver = redirect
  check_local_user
  file = $home/.forward
  allow_filter
  no_verify
  no_expn
  check_ancestor
  file_transport = address_file
  pipe_transport = address_pipe
  reply_transport = address_reply

localuser:
  driver = accept
  check_local_user
  #transport = local_delivery
  transport = dovecot_delivery
  cannot_route_message = Unknown user

######################################################################
#                      TRANSPORTS CONFIGURATION                      #
######################################################################
#                       ORDER DOES NOT MATTER                        #
#     Only one appropriate transport is called for each delivery.    #
######################################################################

begin transports

remote_smtp:
  driver = smtp

local_delivery:
  driver = appendfile
  file = /var/mail/$local_part
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail

address_pipe:
  driver = pipe
  return_output

address_file:
  driver = appendfile
  delivery_date_add
  envelope_to_add
  return_path_add
  mode = 0660

address_reply:
  driver = autoreply

dovecot_delivery:
  driver = pipe
  command = /usr/local/libexec/dovecot/deliver -d $local_part@$domain
  message_prefix =
  message_suffix =
  delivery_date_add
  envelope_to_add
  return_path_add
  log_output
  user = vmail