apt install postfix -y
Internetmail with Smart Host
FQDN meinekleinefarm.net
Relayhost external-relay.de
 
cp /etc/postfix/main.cf /etc/postfix/main.cf.BAK
nano /etc/postfix/main.cf

• smtpd_tls_cert_file=/etc/letsencrypt/live/meinekleinefarm.net/fullchain.pem
  smtpd_tls_key_file=/etc/letsencrypt/live/meinekleinefarm.net/privkey.pem
  myhostname = mail.meinekleinefarm.net
  # mydestination = debian.meinekleinefarm.net, localhost.meinekleinefarm.net, localhost
   
  # Handing off local delivery to Dovecot's LMTP
  virtual_transport = lmtp:unix:private/dovecot-lmtp
   
  # Enabling SMTP for authenticated users, and handing off authentication to Dovecot
  smtpd_sasl_type = dovecot
  smtpd_sasl_path = private/auth
  smtpd_sasl_auth_enable = yes
  smtpd_tls_auth_only = yes
   
  # Virtual domains, users, and aliases
  virtual_mailbox_domains = /etc/postfix/virtual_mailbox_domains
  # virtual_mailbox_maps = /etc/postfix/virtual_mailbox_maps
  # virtual_alias_maps = hash:/etc/postfix/virtual_alias_maps
  # virtual_alias_maps = regexp:/etc/postfix/virtual_alias_global
   
  # relay_recipient_maps = hash:/etc/postfix/relay_recipients

nano /etc/postfix/virtual_mailbox_domains

• meinekleinefarm.net OK
  mgs-ullrich.de OK

postmap /etc/postfix/virtual_mailbox_domains
 
cp /etc/postfix/master.cf /etc/postfix/master.cf.BAK
nano /etc/postfix/master.cf

• Uncomment Line 17 to activate secure message submission agent (MSA) on port 587
  ''submission inet n       -       -       -       -       smtpd''

systemctl restart postfix