Postfix/dovecot use external smtp server to send auto-reply

Question

I want that dovecot vacation extension uses an external smtp server for sending the out of office (vacation) mails.

Current setup: I use postfix for my incomming mails and dovecot for my imap, all this runs in k8s. One deployment with postfix and one deployment with dovecot.

Incomming host is mail.domain.nl Outgoing (smtp) is smtp.domain.nl

so a mail comes with postfix then goes to dovecot, using aliases and there i have a config file that says when "to" is [email protected] send vacation reply

I have researched some time and found that with dovecot you can specify an external smtp with

submission_host = smtp.domain.nl:587

When testing this i got an authentication error:

vacation action: failed to send vacation response to X: <smtp(mail.domain.nl:587): RCPT TO failed: 554 5.7.1 You are not authenticated It is only possible to submit mails over secure authenticated connections Please secure the connection with STARTTL and send a valid username and password before you send "MAIL FROM"> (permanent error)

Is it possible to set authentication for the external smtp? Or can i maybe use postfix to send the replies? If so how?

postfix.conf

postscreen_upstream_proxy_protocol = haproxy
postscreen_upstream_proxy_timeout = 5s    
alias_maps = hash:/etc/postfix/aliases
alias_database = hash:/etc/postfix/aliases
smtpd_tls_cert_file=/etc/postfix/ssl/tls.crt
smtpd_tls_key_file=/etc/postfix/ssl/tls.key
smtpd_tls_security_level=may

smtp_tls_CApath=/etc/ssl/certs
smtp_tls_security_level=may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
mydestination = hash:/etc/postfix/mydestinations
relayhost = 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command =
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = ipv4
virtual_alias_domains = proxy:mysql:/etc/postfix/mysql_virtual_alias_domains.cf,hash:/etc/postfix/local_recipient_maps
virtual_alias_maps = hash:/etc/postfix/local_recipient_maps,mysql:/etc/postfix/mysql_virtual_alias_maps.cf,proxy:mysql:/etc/postfix/mysql_forwards.cf
virtual_mailbox_domains = virtual.tld
virtual_transport = lmtp:dovecot.mail.svc.cluster.local:24
virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql_virtual_mailbox_maps.cf
spamc_destination_recipient_limit = 1

smtp      inet  n       -       n       -       1       postscreen
smtpd     pass  -       -       n       -       -       smtpd -o content_filter=spamc
pickup    unix  n       -       n       60      1       pickup
cleanup   unix  n       -       n       -       0       cleanup
qmgr      unix  n       -       n       300     1       qmgr
#qmgr     unix  n       -       n       300     1       oqmgr
tlsmgr    unix  -       -       n       1000?   1       tlsmgr
rewrite   unix  -       -       n       -       -       trivial-rewrite
bounce    unix  -       -       n       -       0       bounce
defer     unix  -       -       n       -       0       bounce
trace     unix  -       -       n       -       0       bounce
verify    unix  -       -       n       -       1       verify
flush     unix  n       -       n       1000?   0       flush
proxymap  unix  -       -       n       -       -       proxymap
proxywrite unix -       -       n       -       1       proxymap
smtp      unix  -       -       n       -       -       smtp
relay     unix  -       -       n       -       -       smtp
        -o syslog_name=postfix/$service_name
#       -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
showq     unix  n       -       n       -       -       showq
error     unix  -       -       n       -       -       error
retry     unix  -       -       n       -       -       error
discard   unix  -       -       n       -       -       discard
local     unix  -       n       n       -       -       local
virtual   unix  -       n       n       -       -       virtual
lmtp      unix  -       -       n       -       200     lmtp
anvil     unix  -       -       n       -       1       anvil
scache    unix  -       -       n       -       1       scache
postlog   unix-dgram n  -       n       -       1       postlogd

spamc     unix  -       n       n       -       200     pipe
    flags=DROhu  user=nobody argv=/usr/bin/spamc -u ${user} -d spamassassin.mail -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}

dovecot.conf

haproxy_trusted_networks = 10.40.0.0/16
service imap-login {
  inet_listener imap {
    haproxy = yes
  }
  inet_listener imaps {
    haproxy = yes
  }
}
service pop3-login {
  inet_listener pop3 {
    haproxy = yes
  }
  inet_listener pop3s {
    haproxy = yes
  }
}
service lmtp {
  executable = lmtp -L
  inet_listener {
    port = 24
  }
}
protocol lmtp {
  postmaster_address = [email protected]
  info_log_path = /dev/stdout
  mail_plugins = sieve
}

submission_host = smtp.domain.nl:587

lda_original_recipient_header = X-Original-To
plugin {
      sieve = /sieve/default.sieve
      sieve_vacation_use_original_recipient = yes
    }

# default.sieve
require ["vacation"];
if header :contains "To" ["X"] {
vacation 
            :days 1
            :subject "Out of Office AutoReply"
            "This message is to inform you that I am out of the office.";