Mail server seems to be working but mail client does not show mails

Question

for the last few days I have been trying to run an email server and it looks like I finally made it.

The email server runs on Debian in combination with postfix and dovecot. Webmin is also installed, in which I see emails for an user and I can also send emails.

I created a test user "Kontakt" and here is a screenshot from the webmin SCREENSHOT

Next I installed roundcube, configured and tested the smtp and imap connection. Everything is fine, but after logging in to the roundcube I do not see any emails.

I found that emails are stored in /var/spool/kontakt, but not in maildir and I do not why.

Summary - My goal is to be able to write, read and manage emails via a roundcube.

QUESTION 2: How can I ensure that an email address can only have a domain name, not a hostname?

For example, if I send an email from gmail to kontakt@mail.mintiga.cz, the email is ok. But if I send an email to kontakt@mintiga.cz, I will receive a message back that the email could not be delivered.

Thank you very much

     <kontakt@mintiga.cz>: User unknown in virtual alias table

postconf: warning: /etc/postfix/master.cf: undefined parameter: mua_sender_restrictions
postconf: warning: /etc/postfix/master.cf: undefined parameter: mua_helo_restrictions
alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
allow_min_user = yes
append_dot_mydomain = no
biff = no
compatibility_level = 2
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = ipv4
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
mydestination = $myhostname, xxxxxx.contaboserver.net, localhost.contaboserver.net, , localhost
mydomain = mintiga.cz
myhostname = mail.mintiga.cz
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
myorigin = $mydomain
queue_directory = /var/spool/postfix
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_tls_loglevel = 1
smtp_tls_note_starttls_offer = yes
smtp_tls_security_level = may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_recipient_restrictions =     permit_mynetworks,permit_auth_destination,permit_sasl_authenticated,reject
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
smtpd_sasl_auth_enable = yes
smtpd_sasl_path = private/auth
smtpd_sasl_security_options = noanonymous
smtpd_sasl_type = dovecot
smtpd_tls_cert_file = /etc/letsencrypt/live/mail.mintiga.cz/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/mail.mintiga.cz/privkey.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes
virtual_alias_domains = $mydomain
virtual_alias_maps = hash:/etc/postfix/virtual
postconf: warning: /etc/postfix/main.cf: unused parameter: smtpd_sasl_location_domain=$myhostname


# 2.2.27 (): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.16 ()
# OS: Linux 4.9.0-15-amd64 x86_64 Debian 9.13
auth_debug = yes
auth_debug_passwords = yes
auth_mechanisms = plain login
auth_verbose = yes
disable_plaintext_auth = no
mail_debug = yes
mail_location = maildir:~/Maildir
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
  separator = .
  type = private
}
passdb {
  driver = pam
}
protocols = " imap pop3"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
service imap-login {
  inet_listener imap {
    port = 143
  }
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
ssl = required
ssl_cert = </etc/letsencrypt/live/mail.mintiga.cz/fullchain.pem
ssl_key =  # hidden, use -P to show it
userdb {
  driver = passwd
}