How can I bulk check and clean MX records for a list of domains?

example.com
example.net
example.org
example.invalid

dig -t MX -f domains.txt +noall +comments +question +answer > mx-answers.txt

example.com. 3600 IN MX 10 mx1.example.net.
example.org. 3600 IN MX 0 .

#!/usr/bin/env bash
set -u

if [ "$#" -gt 0 ]; then
  input="$1"
else
  input=domains.txt
fi

work_file=$(mktemp)
trap 'rm -f "$work_file"' EXIT

normalize_domain() {
  sed -E '
    s/^[[:space:]]+//;
    s/[[:space:]]+$//;
    s#^[A-Za-z]+://##;
    s#^[^@/]+@##;
    s#/.*$##;
    s/:[0-9]+$//;
    s/\.$//
  ' | tr '[:upper:]' '[:lower:]'
}

while IFS= read -r raw; do
  printf '%s\n' "$raw" | normalize_domain
done < "$input" | awk 'NF && !seen[$0]++' > "$work_file"

dns_status() {
  awk '/status:/ {
    gsub(/,/, "", $6)
    print $6
    exit
  }'
}

has_type() {
  record_type="$1"
  awk -v t="$record_type" '$4 == t {found=1} END {exit !found}'
}

emit() {
  printf '"%s","%s","%s","%s"\n' "$1" "$2" "$3" "$4"
}

printf 'domain,status,dns_status,mx_hosts\n'

while IFS= read -r domain; do
  mx_reply=$(dig +time=2 +tries=1 "$domain" MX +noall +comments +answer 2>/dev/null)
  mx_status=$(printf '%s\n' "$mx_reply" | dns_status)

  if [ -z "$mx_status" ]; then
    emit "$domain" retry_timeout timeout ""
    continue
  fi

  if [ "$mx_status" = NXDOMAIN ]; then
    emit "$domain" nxdomain NXDOMAIN ""
    continue
  fi

  if [ "$mx_status" != NOERROR ]; then
    lower_status=$(printf '%s' "$mx_status" | tr '[:upper:]' '[:lower:]')
    emit "$domain" "retry_$lower_status" "$mx_status" ""
    continue
  fi

  mx_rows=$(printf '%s\n' "$mx_reply" | awk '$4 == "MX" {print $5, $6}')
  mx_count=$(printf '%s\n' "$mx_rows" | awk 'NF {n++} END {print n+0}')
  valid_null=$(printf '%s\n' "$mx_rows" |
    awk '$1 == 0 && $2 == "." {n++} END {print n+0}')
  any_dot=$(printf '%s\n' "$mx_rows" |
    awk '$2 == "." {n++} END {print n+0}')

  if [ "$mx_count" -eq 1 ] && [ "$valid_null" -eq 1 ]; then
    emit "$domain" null_mx NOERROR "0:."
    continue
  fi

  if [ "$any_dot" -gt 0 ]; then
    emit "$domain" malformed_mx NOERROR "mixed_null_mx"
    continue
  fi

  if [ "$mx_count" -gt 0 ]; then
    mx_hosts=$(printf '%s\n' "$mx_rows" | awk '
      {
        host=$2
        sub(/\.$/, "", host)
        printf "%s%s:%s", sep, $1, host
        sep=";"
      }
    ')
    usable=false
    transient=false
    malformed=false

    while read -r preference host; do
      host=$(printf '%s' "$host" | sed 's/\.$//')
      a_reply=$(dig +time=2 +tries=1 "$host" A +noall +comments +answer 2>/dev/null)
      aaaa_reply=$(dig +time=2 +tries=1 "$host" AAAA +noall +comments +answer 2>/dev/null)
      a_status=$(printf '%s\n' "$a_reply" | dns_status)
      aaaa_status=$(printf '%s\n' "$aaaa_reply" | dns_status)

      if printf '%s\n' "$a_reply" | has_type CNAME ||
        printf '%s\n' "$aaaa_reply" | has_type CNAME
      then
        malformed=true
      fi

      if printf '%s\n' "$a_reply" | has_type A ||
        printf '%s\n' "$aaaa_reply" | has_type AAAA
      then
        usable=true
      fi

      if { [ -z "$a_status" ] || { [ "$a_status" != NOERROR ] && [ "$a_status" != NXDOMAIN ]; }; } ||
        { [ -z "$aaaa_status" ] || { [ "$aaaa_status" != NOERROR ] && [ "$aaaa_status" != NXDOMAIN ]; }; }
      then
        transient=true
      fi
    done <<EOF
$mx_rows
EOF

    if [ "$malformed" = true ]; then
      emit "$domain" malformed_mx NOERROR "$mx_hosts"
    elif [ "$usable" = true ]; then
      emit "$domain" has_usable_mx NOERROR "$mx_hosts"
    elif [ "$transient" = true ]; then
      emit "$domain" retry_dns_error temporary "$mx_hosts"
    else
      emit "$domain" unusable_mx NOERROR "$mx_hosts"
    fi
    continue
  fi

  a_reply=$(dig +time=2 +tries=1 "$domain" A +noall +comments +answer 2>/dev/null)
  aaaa_reply=$(dig +time=2 +tries=1 "$domain" AAAA +noall +comments +answer 2>/dev/null)
  a_status=$(printf '%s\n' "$a_reply" | dns_status)
  aaaa_status=$(printf '%s\n' "$aaaa_reply" | dns_status)

  if printf '%s\n' "$a_reply" | has_type A ||
    printf '%s\n' "$aaaa_reply" | has_type AAAA
  then
    emit "$domain" implicit_mx NOERROR ""
  elif { [ -z "$a_status" ] || { [ "$a_status" != NOERROR ] && [ "$a_status" != NXDOMAIN ]; }; } ||
    { [ -z "$aaaa_status" ] || { [ "$aaaa_status" != NOERROR ] && [ "$aaaa_status" != NXDOMAIN ]; }; }
  then
    emit "$domain" retry_dns_error temporary ""
  else
    emit "$domain" no_mail_route NOERROR ""
  fi
done < "$work_file"