When attempting to send emails to t-online.de recipients, many servers encounter this rejection:

host mx02.t-online.de[194.25.134.9] refused to talk to me: 
554 IP=xx.xx.xx.xx - A problem occurred. (Ask your postmaster
for help or to contact tosa@rx.t-online.de to clarify.) (BL)

The critical distinction in your case is the (BL) suffix, which indicates:

• This isn't a temporary volume-based block (their standard 100 emails/day limit)
• Your IP has been manually blacklisted by t-online.de's abuse team
• Automatic unblocking won't occur after 24 hours

Here's how to diagnose the issue programmatically:

# Check if your IP is listed in common RBLs
dig +short xx.xx.xx.xx.bl.spamcop.net
dig +short xx.xx.xx.xx.zen.spamhaus.org

# Verify your SMTP headers
telnet mx02.t-online.de 25
EHLO yourdomain.com
MAIL FROM: <validuser@yourdomain.com>
RCPT TO: <testuser@t-online.de>

Based on experience with t-online.de's filters:

1. Historical reputation issues: IP previously used for spam
2. Forwarding chains: Even if you don't forward, recipients might
3. Content filtering: Certain keywords trigger blocks
4. Missing/broken reverse DNS

For Python SMTP implementations:

import smtplib
from email.mime.text import MIMEText

msg = MIMEText("Test email body")
msg['Subject'] = 'Test Subject'  # Avoid spammy keywords
msg['From'] = 'verified@yourdomain.com'
msg['To'] = 'recipient@t-online.de'

try:
    with smtplib.SMTP('your.smtp.server', 587) as server:
        server.starttls()
        server.login('user', 'password')
        server.send_message(msg)
except smtplib.SMTPResponseException as e:
    if e.smtp_code == 554 and '(BL)' in e.smtp_error:
        print("Blacklisted - contact tosa@rx.t-online.de")

Contact t-online.de's postmaster with:

• Your server IP and domain
• Sample message headers that were blocked
• SPF/DKIM/DMARC configuration details
• Any relevant mail logs (sanitized)

While resolving the BL issue, consider:

# Route through authenticated SMTP relay
import smtplib
s = smtplib.SMTP('smtp.mailgun.org', 587)
s.login('postmaster@yourdomain.mailgun.org', 'password')
s.sendmail(from_addr, 'user@t-online.de', msg.as_string())

html

When our mail server attempted delivery to t-online.de addresses, we encountered:

host mx02.t-online.de[194.25.134.9] refused to talk to me: 
554 IP=xx.xx.xx.xx - A problem occurred. 
(Ask your postmaster for help or contact tosa@rx.t-online.de to clarify.) (BL)

Unlike the well-documented 100-messages/day threshold (without (BL) suffix), this appears to be a separate blocking mechanism with these characteristics:

• Persists beyond 24 hours (observed 72+ hours in our case)
• Not correlated with spamhaus or other RBL listings
• Triggered even with legitimate mail volumes (20-30/day in our case)

Here's how we diagnosed the issue:

# Check IP reputation
curl -s "https://api.blacklistcheck.org/check?ip=YOUR_IP"

# Verify DNS-based blocklists
dig +short YOUR_IP.dnsbl.example.org

# Test SMTP connectivity manually
telnet mx02.t-online.de 25
EHLO yourdomain.com
MAIL FROM: <validuser@yourdomain.com>
RCPT TO: <testuser@t-online.de>

Through header analysis of bounced messages, we discovered:

X-T-Online-Mail: Rejected by policy (BL-2014.01)
X-T-Online-Info: https://postmaster.t-online.de

The policy document reveals "BL" indicates their internal blacklist, distinct from:

• Volume-based throttling (no suffix)
• Spamhaus blocks (SH)
• DNSBL hits (DNS)

Effective remediation requires:

1. Formal delisting request to tosa@rx.t-online.de containing:

   Subject: IP Delisting Request for [YOUR_IP]
   
   Body:
   - Full server IP address
   - Reverse DNS record
   - Sample message headers
   - SPF/DKIM/DMARC configuration

2. Technical adjustments:

   # Postfix configuration example
   smtp_helo_name = mail.yourdomain.com
   smtpd_banner = $myhostname ESMTP $mail_name
   
   # Ensure proper PTR records
   host YOUR_IP | awk '{print "PTR: "$NF}'
   

For critical systems, consider these SMTP relay options:

# Using Amazon SES as relay
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
relayhost = [email-smtp.us-east-1.amazonaws.com]:587

# SendGrid configuration
smtp_tls_security_level = encrypt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous

Implement these checks to avoid future blocks:

#!/bin/bash
# Daily blocklist monitor
IP=$(curl -s ifconfig.me)
BL_CHECK=$(dig +short $IP.bl.t-email.net)
[ -z "$BL_CHECK" ] || \
echo "ALERT: IP $IP listed in t-online BL" | mail -s "SMTP Alert" admin@domain.com