setup-postfix-outbound.sh

# run like: bash setup-postfix-outbound.sh someone@somewhere.com
#!/usr/bin/env bash
set -euo pipefail

TO="${1:-}"
if [[ -z "${TO}" ]]; then
  echo "Usage: $0 recipient@example.com"
  exit 1
fi

# Re-run as root if needed
if [[ "${EUID}" -ne 0 ]]; then
  exec sudo -E bash "$0" "$TO"
fi

export DEBIAN_FRONTEND=noninteractive

apt-get update -y
apt-get install -y postfix bsd-mailx ca-certificates

# Ensure a main config exists (some broken images/templates may be missing it)
if [[ ! -f /etc/postfix/main.cf ]]; then
  if [[ -f /etc/postfix/main.cf.proto ]]; then
    cp /etc/postfix/main.cf.proto /etc/postfix/main.cf
  else
    # last resort: create a minimal config file
    postconf -n > /etc/postfix/main.cf
  fi
fi

# Remove known "blank value" keys that can crash postfix if they exist as "key ="
for k in debugger_command html_directory manpage_directory sample_directory readme_directory; do
  if grep -Eq "^${k}[[:space:]]*=[[:space:]]*$" /etc/postfix/main.cf; then
    postconf -X "$k" || true
  fi
done

# Required paths/groups (avoids template blanks like setgid_group =)
postconf -e "setgid_group=postdrop"
postconf -e "sendmail_path=/usr/sbin/sendmail"
postconf -e "newaliases_path=/usr/bin/newaliases"
postconf -e "mailq_path=/usr/bin/mailq"

HN="$(hostname -f 2>/dev/null || hostname)"
DM="$(hostname -d 2>/dev/null || true)"

postconf -e "myhostname=${HN}"
if [[ -n "${DM}" ]]; then
  postconf -e "mydomain=${DM}"
fi

# Outbound-only (no listening on public interfaces)
postconf -e "inet_interfaces=loopback-only"
postconf -e "inet_protocols=ipv4"

# Keep local delivery sane (so DSNs don’t try weird remote routes)
postconf -e 'mydestination=$myhostname, localhost.$mydomain, localhost'

systemctl enable --now postfix
postfix check

SUBJ="postfix outbound test $(date -Is)"
BODY="Test email via Postfix from ${HN} at $(date -Is)."

printf "Subject: %s\nTo: %s\nFrom: root@%s\n\n%s\n" \
  "${SUBJ}" "${TO}" "${HN}" "${BODY}" | sendmail -t -v

echo "Sent. If it doesn't arrive, check:"
echo "  journalctl -u postfix -n 80 --no-pager"