The 554 SMTP error code indicating "sending MTA's poor reputation" typically means the receiving mail server has identified your mail transfer agent (MTA) as having questionable sending practices. This isn't necessarily about blacklisting - it's about reputation metrics that many modern email providers calculate independently.

Before making configuration changes, verify these fundamentals:

# Check reverse DNS (should match your hostname)
host 77.245.64.44

# Verify SPF record (example for dig)
dig TXT yourdomain.com

# Check DKIM signing (replace selector as needed)
dig TXT selector._domainkey.yourdomain.com

# Test SMTP conversation (basic check)
telnet mail.example.com 25
EHLO yourdomain.com

Modern email providers use complex algorithms considering:

• Volume patterns (sudden spikes trigger alerts)
• Recipient engagement (high bounce rates hurt scores)
• Content similarity (mass identical emails look spammy)

For Postfix configurations, add these reputation-protecting settings:

# In main.cf
smtpd_client_connection_rate_limit = 100
anvil_rate_time_unit = 60s
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20

If this is a new server IP, implement a gradual warm-up schedule:

# Example sending schedule (emails/day)
Days 1-3: 50-100
Days 4-7: 100-200
Week 2: 200-500
Week 3: 500-1000

Implement these monitoring solutions:

# Sample Nagios check for bounce rates
define service {
    service_description     SMTP Bounce Rate
    check_command           check_smtp_bounce!5%!10%
    max_check_attempts      3
    normal_check_interval   5
    retry_check_interval    1
}

For cloud-based monitoring, configure webhook alerts from services like Mailgun's reputation API:

curl -X GET \
  https://api.mailgun.net/v3/domains/yourdomain.com/stats/total \
  -u 'api:YOUR_API_KEY' \
  -d event='rejected' \
  -d duration='1d'

Modify your MTA to include reputation-boosting headers:

# For Exim configuration
remote_smtp:
  driver = smtp
  headers_add = X-Reputation-Score: ${if exists{/etc/exim_reputation_score}{${readfile{/etc/exim_reputation_score}}}{80}}
  headers_add = X-Mailer-Verified: ${if exists{/etc/exim_verified}{${readfile{/etc/exim_verified}}}{false}}

The "554 Your access to this mail system has been rejected due to the sending MTA's poor reputation" error occurs when receiving mail servers block emails based on the sender's Mail Transfer Agent (MTA) reputation score. This is different from traditional blacklisting and involves more complex reputation systems like:

• Microsoft's Smart Network Data Services (SNDS)
• Google Postmaster Tools reputation
• Cisco Talos Intelligence
• Spamhaus Domain Block List (DBL)

First, verify if your server IP (77.245.64.44) has reputation issues:

# Check basic IP reputation
dig +short 77.245.64.44.zen.spamhaus.org
nslookup 77.245.64.44.bl.score.senderscore.com

# For Microsoft SNDS (requires registration)
curl -s "https://sendersupport.olc.protection.outlook.com/snds/data.aspx?key=YOUR_KEY&ip=77.245.64.44"

# Google Postmaster Tools (requires verification)
# https://postmaster.google.com

The error suggests systemic reputation issues rather than configuration problems. Common causes include:

# 1. Check your bounce rate (Python example)
import re
from collections import Counter

with open('/var/log/mail.log') as f:
    bounces = Counter(re.findall(r'status=bounced', f.read()))
    print(f"Bounce rate: {bounces['status=bounced']}")

For new IPs, implement gradual warm-up:

# PHP example for throttled sending
$recipients = ["a@domain.com", "b@domain.com"];
$max_per_hour = 50; // Start small
$delay = 3600 / $max_per_hour;

foreach ($recipients as $to) {
    mail($to, $subject, $message, $headers);
    sleep($delay);
}

Implement these Postfix configurations in main.cf:

# Rate limiting
smtpd_client_message_rate_limit = 100
anvil_rate_time_unit = 60s

# DKIM signing (Ubuntu/Debian example)
milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

Create a monitoring script (Bash example):

#!/bin/bash
IP=77.245.64.44
TOOLS=(
    "https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a$IP"
    "http://multirbl.valli.org/lookup/$IP.html"
    "https://www.dnsbl.info/$IP"
)

for tool in "${TOOLS[@]}"; do
    echo "Checking $tool"
    curl -s "$tool" | grep -E "not listed|clean"
done

Consider using a SMTP relay service with these API examples:

// Node.js example using Mailgun
const mailgun = require("mailgun-js");
const mg = mailgun({
  apiKey: "YOUR_API_KEY",
  domain: "yourdomain.com",
  host: "api.eu.mailgun.net" // EU region better for GDPR
});

mg.messages().send({
  from: "you@yourdomain.com",
  to: "recipient@example.com",
  subject: "Test",
  text: "Testing Mailgun!"
});