Complete Guide: Fixing "TLS is required, but was not offered" Email Delivery Errors

Email delivery failures can be frustrating, especially when they involve cryptic technical messages. One of the most common modern email delivery errors is the "TLS is required, but was not offered" message. This comprehensive tutorial will walk you through understanding, diagnosing, and resolving this issue step by step.

Understanding the Error Message

What You're Seeing

When you encounter this error, you'll typically receive a bounce-back message similar to this:

This is the mail system at host dal01-us.mx.enginyring.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

<recipient@domain.com>: TLS is required, but was not offered by host
    mail.domain.com[IP.ADDRESS.HERE]

Breaking Down the Error

Let's dissect what each part means:

• Sending server: dal01-us.mx.enginyring.com - This is the outbound mail server trying to deliver your email
• Error type: "TLS is required, but was not offered"
• Receiving server: mail.domain.com[IP.ADDRESS] - The destination server that failed to provide TLS

What is TLS and Why Does It Matter?

Understanding TLS (Transport Layer Security)

TLS is a cryptographic protocol that provides secure communication over computer networks. In email context, it encrypts the connection between mail servers during message transmission.

Key benefits of TLS in email:

• Encryption: Protects email content from interception
• Authentication: Verifies server identities
• Integrity: Ensures messages aren't modified in transit
• Compliance: Meets modern security standards

Why TLS Requirements Are Increasing

Modern email providers like Gmail, Outlook, and others have implemented mandatory TLS policies because:

1. Security mandates: Industry standards now require encrypted email transmission
2. Privacy regulations: GDPR, HIPAA, and other regulations encourage encryption
3. Reputation protection: Helps prevent spam and phishing attacks
4. Data integrity: Ensures messages reach recipients unmodified

Step-by-Step Diagnosis Process

Step 1: Identify the Parties Involved

Before fixing anything, clearly identify:

1. Your sending domain/server (where the email originates)
2. The recipient's domain/server (where delivery failed)
3. Your email service provider (if using hosted email)

Step 2: Determine Your Role

If you're the sender:

• You need to work with the recipient or their IT team
• You may need to adjust your sending server settings
• Consider alternative delivery methods temporarily

If you're the recipient's administrator:

• You need to enable TLS on your mail server
• This is typically your responsibility to fix

Step 3: Test TLS Connectivity

Use these command-line tools to test TLS support:

Testing with OpenSSL:

openssl s_client -connect mail.domain.com:587 -starttls smtp

Testing with Telnet:

telnet mail.domain.com 25

Then type:

EHLO test.com

Look for "STARTTLS" in the response. If missing, TLS isn't supported.

Solution 1: Enabling TLS on Your Mail Server

For System Administrators

If you manage the receiving mail server, follow these steps:

Step 1: Obtain SSL/TLS Certificates

Option A: Free certificates with Let's Encrypt

# Install Certbot
sudo apt-get update
sudo apt-get install certbot

# Generate certificate
sudo certbot certonly --standalone -d mail.yourdomain.com

Option B: Commercial certificates

• Purchase from providers like DigiCert, Comodo, or GoDaddy
• Follow their verification process
• Download the certificate files

Step 2: Configure Postfix (Most Common)

Edit main configuration file:

sudo nano /etc/postfix/main.cf

Add these lines:

# TLS Configuration
smtpd_tls_cert_file = /etc/ssl/certs/mail.yourdomain.com.crt
smtpd_tls_key_file = /etc/ssl/private/mail.yourdomain.com.key
smtpd_use_tls = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_tls_security_level = may
smtp_tls_security_level = may
smtpd_tls_protocols = !SSLv2, !SSLv3

Restart Postfix:

sudo systemctl restart postfix

Step 3: Configure Exim

Edit configuration file:

sudo nano /etc/exim4/conf.d/main/03_exim4-config_tlsoptions

Add TLS settings:

tls_certificate = /etc/ssl/certs/mail.yourdomain.com.crt
tls_privatekey = /etc/ssl/private/mail.yourdomain.com.key
tls_advertise_hosts = *

Update and restart:

sudo update-exim4.conf
sudo systemctl restart exim4

For Hosted Email Services

Step 1: Contact Your Provider

If using hosted email (shared hosting, managed services):

1. Submit a support ticket explaining the TLS requirement
2. Request TLS enablement for your domain
3. Provide any necessary documentation about the error

Step 2: Upgrade Your Plan

Some providers offer TLS only on higher-tier plans:

• Review your current email hosting plan
• Check if TLS is included in your package
• Upgrade if necessary

Solution 2: Sender-Side Adjustments

When You Can't Change the Recipient's Server

Step 1: Modify TLS Requirements (Temporary)

For Postfix users:

sudo nano /etc/postfix/main.cf

Add transport map:

transport_maps = hash:/etc/postfix/transport

Create transport file:

sudo nano /etc/postfix/transport

Add domain-specific rule:

problematic-domain.com    smtp-notls:

Create corresponding service in master.cf:

sudo nano /etc/postfix/master.cf

Add:

smtp-notls unix  -       -       n       -       -       smtp
  -o smtp_tls_security_level=none

Apply changes:

sudo postmap /etc/postfix/transport
sudo systemctl reload postfix

Step 2: Use Alternative Delivery Methods

Email relay services:

• Amazon SES
• SendGrid
• Mailgun
• Postmark

These services handle TLS negotiations and often have better compatibility.

Solution 3: DNS and Firewall Considerations

Step 1: Verify DNS Records

Check MX records:

dig MX domain.com

Ensure proper A records:

dig A mail.domain.com

Step 2: Firewall Configuration

Open necessary ports:

• Port 25 (SMTP)
• Port 587 (Submission with STARTTLS)
• Port 465 (SMTPS if used)

Example iptables rules:

sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 587 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 465 -j ACCEPT

Advanced Troubleshooting

Step 1: Log Analysis

Check Postfix logs:

sudo tail -f /var/log/mail.log

Look for specific patterns:

• Certificate errors
• TLS handshake failures
• Connection timeouts

Step 2: Certificate Validation

Test certificate validity:

openssl x509 -in /path/to/certificate.crt -text -noout

Check expiration date:

openssl x509 -in certificate.crt -noout -dates

Step 3: Network Connectivity Tests

Test basic connectivity:

telnet mail.domain.com 25

Test with specific TLS versions:

openssl s_client -connect mail.domain.com:587 -starttls smtp -tls1_2

Prevention and Best Practices

Regular Maintenance

Monthly Tasks

1. Monitor certificate expiration dates
2. Review mail server logs for errors
3. Test email delivery to various providers
4. Update mail server software

Quarterly Tasks

1. Review TLS cipher suites
2. Update SSL/TLS certificates if needed
3. Audit email security policies
4. Test disaster recovery procedures

Security Hardening

Strong TLS Configuration

Recommended cipher suites:

smtpd_tls_mandatory_ciphers = high
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

Certificate Best Practices

1. Use wildcard certificates for multiple subdomains
2. Implement certificate pinning where possible
3. Monitor certificate transparency logs
4. Set up automated renewal for Let's Encrypt certificates

Monitoring and Alerting

Set Up Monitoring

Nagios check example:

/usr/lib/nagios/plugins/check_smtp -H mail.domain.com -p 587 -S

Zabbix template for SMTP TLS monitoring

Log Analysis Tools

• ELK Stack (Elasticsearch, Logstash, Kibana)
• Splunk for enterprise environments
• Graylog for centralized logging

Common Pitfalls and How to Avoid Them

Certificate Issues

Problem: Self-signed certificates Solution: Use properly signed certificates from recognized CAs

Problem: Expired certificates Solution: Implement automated renewal and monitoring

Configuration Errors

Problem: Mixed TLS policies Solution: Consistent configuration across all mail servers

Problem: Firewall blocking TLS ports Solution: Proper firewall configuration and testing

Compatibility Issues

Problem: Outdated TLS versions Solution: Support modern TLS versions (1.2+)

Problem: Weak cipher suites Solution: Configure strong, modern cipher suites

Testing Your Solution

Step 1: Basic Functionality Test

Send test emails to:

• Gmail accounts
• Outlook/Hotmail accounts
• Yahoo accounts
• Other email providers

Step 2: External Testing Tools

Use online tools:

• MX Toolbox SMTP Test
• Mail-tester.com
• SSL Labs Email Test

Step 3: Command Line Verification

Test TLS connection:

openssl s_client -connect yourserver.com:587 -starttls smtp

Verify certificate chain:

openssl s_client -showcerts -connect yourserver.com:587 -starttls smtp

When to Seek Professional Help

Indicators You Need Expert Assistance

1. Persistent certificate errors despite following guides
2. Complex enterprise email environments
3. Compliance requirements (HIPAA, SOX, PCI-DSS)
4. Multiple mail servers requiring coordination
5. Custom applications integrated with email systems

Finding the Right Help

• System administrators with email server experience
• Managed service providers specializing in email
• Cloud migration specialists for hosted solutions
• Security consultants for compliance requirements

Conclusion

The "TLS is required, but was not offered" error is a clear indication that email security standards are evolving. While it may seem daunting initially, resolving this issue typically involves enabling TLS support on the receiving mail server through proper certificate installation and configuration.

Remember that implementing TLS is not just about fixing immediate delivery issues. It's about ensuring secure, reliable email communication in an increasingly security-conscious digital landscape. Regular maintenance, monitoring, and staying current with best practices will help prevent future issues and maintain robust email delivery.

By following this guide systematically, you should be able to diagnose and resolve TLS-related email delivery problems effectively. When in doubt, don't hesitate to seek professional assistance, especially in enterprise environments where email reliability is critical to business operations.