# adduser --home /srv/mail-relay --system --disabled-password --shell /bin/sh mail-relay
# /etc/exim4/conf.d/router/150_local_relayed_domains
relay_to_file:
  debug_print = "R: relay_to_file for $local_part@$domain"
  driver = redirect
  domains = +relay_to_domains
  file_transport = address_file
  # data = /srv/mail-relay/$domain
  data = /srv/mail-relay/mail
  user = mail-relay
  group = mail
# update-exim4.conf

# exim -d -bt test@jo-so.de
# exim -odf -N test@jo-so.de <<<$'Subject: Test\n\nTEST\n.'

Auf dem Laptop musste ich nur bei dpkg-reconfigure exim4-config meine Domain jo-so.de in die Liste der anderen Empfänger-Domains (dc_other_hostnames in /etc/exim4/update-exim4.conf.conf) eintragen und schon kamen die E-Mails in mein Postfach.

TLS

adduser Debian-exim ssl-cert

SPF

apt install spf-tools-perl