User at domain.tld doesn't exist (postfix error 550 5.1.1)

[soko.tica]

Hello list,

I am trying to send an e-mail message from the local root to another local
user (alias to root) on the same box. While it functions without dovecot,
it doesn't work with dovecot installed. Local user is recognized, but local
user at domain.tld isn't. While this option is turned on in postfix, I believe
it has something with dovecot configuration, due to doveadm isnt' able to
recognise user at domain.tld

Bellow are the outputs of doveadm user username, doveadm user
username at domain.tld, doveconf -n, tail -f /var/log/mail.log, cat
/etc/aliases and postconf -n.

Please advise. Thanks in advance.

# doveadm user testuser
field    value
uid    1000
gid    1000
home    /home/testuser
mail    maildir:~/Maildir
system_groups_user    testuser
========================================
# doveadm user testuser at mail.domain.tld
field    valueuserdb lookup: user testuser at mail.domain.tld doesn't exist
======================================
# doveadm user testuser at domain.tld
field    valueuserdb lookup: user testuser at domain.tld doesn't exist
=======================================
# doveconf -n
# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.18.26-x1-64 x86_64 Debian 8.3
hostname = mail.domain.tld
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
lda_original_recipient_header = X-Original-To
lmtp_save_to_detail_mailbox = yes
mail_location = maildir:~/Maildir
maildir_very_dirty_syncs = yes
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 =
  separator = /
  type = private
}
passdb {
  driver = pam
}
passdb {
  driver = pam
}
plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
}
postmaster_address = postmaster at mail.domain.tld
protocols = lmtp
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
ssl = no
ssl_cert = </etc/dovecot/dovecot.pem
ssl_key = </etc/dovecot/private/dovecot.pem
userdb {
  driver = passwd
}
userdb {
  driver = passwd
}
protocol lmtp {
  mail_fsync = optimized
}
protocol lda {
  mail_fsync = optimized
}
===============================
# tail -f /var/log/mail.log

Feb 22 13:21:01 uidigaa4 postfix/qmgr[4657]: 75E0722137: removed
Feb 22 13:21:01 uidigaa4 dovecot: lmtp(4764): Connect from local
Feb 22 13:21:01 uidigaa4 postfix/lmtp[4763]: 82EA222140:
to=<testuser at mail.domain.tld>, orig_to=<root at mail.domain.tld>,
relay=mail.domain.tld[private/dovecot-lmtp], delay=0.01, delays=0/0/0.01/0,
dsn=5.1.1, status=bounced (host mail.domain.tld[private/dovecot-lmtp] said:
550 5.1.1 <testuser at mail.domain.tld> User doesn't exist:
testuser at mail.domain.tld (in reply to RCPT TO command))
Feb 22 13:21:01 uidigaa4 dovecot: lmtp(4764): Disconnect from local:
Successful quit
Feb 22 13:21:01 uidigaa4 postfix/qmgr[4657]: 82EA222140: removed
=======================
# cat /etc/aliases
# See man 5 aliases for format
postmaster:    root
root: testuser
===================
# postconf -n
alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
config_directory = /etc/postfix
delay_warning_time = 4h
inet_interfaces = all
mailbox_size_limit = 0
mailbox_transport = lmtp:unix:private/dovecot-lmtp
mydestination = $myhostname localhost.$mydomain localhost $mydomain
mydomain = domain.tld
myhostname = mail.domain.tld
mynetworks = [::ffff:127.0.0.0]/104 [::1]/128
myorigin = $myhostname
notify_classes = resource, software
readme_directory = no
recipient_delimiter = +
relay_domains = $mydestination
relay_transport = error
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes