Learn how to fix the error using Sendmail: Authentication Failed. Our Sendmail Support team is here to help you with your questions and concerns.

“Error Using Sendmail: Authentication Failed”| Fixed

Did you know that the Sendmail error “Authentication failed” occurs when the `sendmail` command cannot authenticate with the SMTP server?

As a result, this prevents emails from being sent, which can disrupt workflows and automated scripts relying on sendmail for notifications.

Sendmail is a popular utility for sending emails from the command line. Authentication failures often stem from issues like incorrect credentials, misconfigurations, or SMTP server restrictions.

Today we are going to explore the different causes behind the Authentication failed error and how to fix them.

An Overview:

• Common Symptoms of the Error
• Common Causes of the “Authentication Failed” Error
• How to Fix the “Sendmail Authentication Failed” Error
• 1. Verify SMTP Credentials
• 2. Configure Authentication in Sendmail
• 3. Verify SMTP Server Hostname and Port
• 4. Check SSL/TLS Configuration
• 5. Verify the Authentication Mechanism
• 6. Check the Sendmail Logs
• Additional Tips to Avoid Future Issues
• Testing Sendmail Configuration
• Troubleshooting Network Issues
  

Common Symptoms of the Error

Recognizing the symptoms of a Sendmail authentication failure helps users identify the issue quickly:

The most direct symptom is the error message itself:

Error using sendmail (line 171): Authentication failed

Here are some of the other symptoms:

• Outgoing emails fail to send, with recipients not receiving messages.
• Bounce-back emails or Delivery Status Notification (DSN) reports may
  appear.
• Checking /var/log/mail.log or /var/log/maillog reveals repeated authentication errors or rejections from the SMTP server.
• The server might return specific SMTP error codes, such as 535 Authentication failed or 550 Authentication required.
• Emails may remain queued in the Sendmail system due to repeated authentication failures.
• High retry counts in the Sendmail queue could indicate a deeper problem.
• If incorrect credentials are used too often, SMTP accounts might become temporarily locked.

Common Causes of the “Authentication Failed” Error

• The provided username or password may be mistyped or outdated.
• Incorrect hostname or port leads to connection failures.
• Missing configurations for SSL/TLS encryption may block authentication.
• Also, missing or incorrect options in `sendmail.mc` or `sendmail.cf` files.
• Additionally, SMTP authentication may not be enabled in Sendmail.
• The SMTP server may not support the authentication method (e.g., PLAIN, LOGIN, or CRAM-MD5) configured in Sendmail.
• Also, restrictions like IP whitelisting or OAuth-only authentication may block Sendmail.
• Furthermore, firewall rules or blocked ports may prevent connections to the SMTP server.

How to Fix the “Sendmail Authentication Failed” Error

1. Verify SMTP Credentials

1. First, confirm that the username and password for the SMTP server are correct.
2. Then, we can test credentials using a tool like OpenSSL:

   openssl s_client -starttls smtp -crlf -connect smtp.example.com:587

3. After connecting, use SMTP commands to authenticate and verify the credentials.

2. Configure Authentication in Sendmail

Sendmail requires proper setup for SMTP authentication in its configuration files.

1. We can enable SMTP Authentication in `sendmail.mc by adding these lines to configure authentication mechanisms:

   
define(`confAUTH_MECHANISMS', `PLAIN LOGIN')dnl
TRUST_AUTH_MECH(`PLAIN LOGIN')dnl
define(`confCACERT_PATH', `/etc/ssl/certs')dnl
define(`confCACERT', `/etc/ssl/certs/ca-certificates.crt')dnl
define(`SMART_HOST', `smtp.your-smtp-server.com')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo.db')dnl

2. Then, create an `authinfo` file. We can store credentials in `/etc/mail/authinfo`:

   AuthInfo:smtp.example.com "U:root" "I:username@example.com" "P:yourpassword" "M:PLAIN"

3. Next, compile the `authinfo` file:

   makemap hash /etc/mail/authinfo < /etc/mail/authinfo
4. Then, rebuild Sendmail configuration:

   m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
systemctl restart sendmail


3. Verify SMTP Server Hostname and Port

Ensure Sendmail connects to the correct SMTP server and port. Common ports are:

• 25: Non-encrypted communication.
• 465: SSL encryption.
• 587: STARTTLS encryption.

Set the correct values in `sendmail.mc` or specify them in the `sendmail` command.

4. Check SSL/TLS Configuration

Some SMTP servers require encryption. Enable STARTTLS or SSL in Sendmail:

• Ensure certificates are in place (`/etc/ssl/certs`).
• Alos, verify STARTTLS is enabled in the `sendmail.mc` configuration.

5. Verify the Authentication Mechanism

Ensure that the authentication method configured in Sendmail matches the SMTP server’s supported methods. Then, update `sendmail.mc` if necessary:

define(`confAUTH_MECHANISMS', `PLAIN LOGIN CRAM-MD5')dnl


6. Check the Sendmail Logs

Then, examine logs for detailed error messages. The logs for the Debian-based systems are at `/var/log/mail.log` () and for the Red Hat-based systems are at `/var/log/maillog` ().

Look for specific errors like failed connections, unsupported authentication methods, or rejected credentials.

Additional Tips to Avoid Future Issues

• Regularly back up `sendmail.mc`, `sendmail.cf`, and `authinfo` files.
• Furthermore, set up alerts for authentication or connection failures.
• After modifying configurations, always test with a sample email to ensure functionality.
• Also, avoid outdated mechanisms like PLAIN when possible; opt for OAuth or TLS for better security.

Testing Sendmail Configuration

Testing our Sendmail setup ensures all configurations work as intended:

• Use the verbose flag to observe the process:

  sendmail -v recipient@example.com

  Review output for clues about connection and authentication issues.
• Then, inspect sendmail.mc and sendmail.cf. Verify key settings, such as SMART_HOST, authentication mechanisms, and encryption requirements.
• Furthermore, use OpenSSL to connect directly to the SMTP server and test credentials:

  openssl s_client -starttls smtp -connect smtp.example.com:587
• Authenticate manually using SMTP commands (EHLO, AUTH, etc.).
• Also, confirm that the authinfo file exists and is properly compiled into authinfo.db.
• Ensure changes to sendmail.mc are reflected in sendmail.cf by running:

  m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
• Finally, restart the service to apply changes:

  systemctl restart sendmail

  Thorough testing minimizes misconfigurations and expedites troubleshooting.

Troubleshooting Network Issues

Network issues can often block Sendmail from connecting to the SMTP server:

• Confirm the server is reachable:

  ping smtp.example.com
• Additionally, use Telnet or Netcat to ensure SMTP ports are open:

  telnet smtp.example.com 587

  Common SMTP ports include 25 (non-encrypted), 465 (SSL), and 587 (STARTTLS).
• Ensure the system firewall allows outbound connections to the SMTP server.
• Update firewall rules if necessary:

  ufw allow out to smtp.example.com port 587
• Ensure the SMTP server’s hostname resolves to the correct IP address:

  nslookup smtp.example.com
• If behind a proxy, confirm that it permits SMTP traffic.
• On multi-homed systems or NAT environments, verify that Sendmail uses the correct network interface.

[Need assistance with a different issue? Our team is available 24/7.]

Conclusion

The “Sendmail Authentication Failed” error disrupts email workflows but can be resolved by systematically verifying credentials, configuring Sendmail properly, and ensuring compatibility with the SMTP server.

In brief, our Support Experts demonstrated how to fix the error using Sendmail: Authentication Failed.