SMTP Troubleshooting Guide

Comprehensive guide to diagnosing and fixing SMTP problems. Learn how to troubleshoot common SMTP errors, connection issues, authentication failures, and email delivery problems.

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Common SMTP Errors and Solutions
Most frequent SMTP problems and how to fix them

Connection Refused / Connection Timeout

Error Code: ECONNREFUSED, Connection timed out

Possible Causes:

  • •SMTP server is down or not responding
  • •Firewall blocking the SMTP port (25, 587, 465)
  • •Incorrect SMTP server hostname or IP address
  • •ISP blocking outbound SMTP connections
  • •Wrong port number specified

Solutions:

  • Verify server address: Double-check the SMTP hostname (e.g., smtp.gmail.com)
  • Check port number: Use 587 (STARTTLS), 465 (SSL), or 25 (rarely used)
  • Test connectivity: Use telnet or ping to verify network connectivity
  • Check firewall: Ensure outbound connections on SMTP ports are allowed
  • Try different network: Test from another network to rule out ISP blocking

Authentication Failed

Error Code: 535, 535-5.7.8, Authentication credentials invalid

Possible Causes:

  • •Incorrect username or password
  • •App-specific password required (Gmail, Microsoft)
  • •Two-factor authentication not configured properly
  • •Wrong username format (some require full email, others just local part)
  • •Account locked or disabled

Solutions:

  • Verify credentials: Double-check username and password are correct
  • Gmail: Enable 2FA and generate an app-specific password
  • Office 365: Use full email address as username
  • Check account status: Ensure account is active and not locked
  • Test authentication: Try logging into webmail with same credentials

Relay Access Denied

Error Code: 554, 550, Relay access denied

Possible Causes:

  • •SMTP server requires authentication but none provided
  • •IP address not whitelisted for relay
  • •Relay permissions not configured on server
  • •Sending to external domain without proper permissions

Solutions:

  • Enable authentication: Provide valid SMTP credentials
  • Whitelist IP: Add your server IP to relay whitelist
  • Configure relay: Update SMTP server relay settings
  • Use authenticated SMTP: Switch to authenticated relay service

SSL/TLS Certificate Errors

Error: Certificate verification failed, SSL handshake failed

Possible Causes:

  • •Expired SSL/TLS certificate
  • •Self-signed certificate not trusted
  • •Certificate hostname mismatch
  • •Wrong encryption method (SSL vs STARTTLS)
  • •Outdated SSL/TLS protocol version

Solutions:

  • Update certificate: Renew expired SSL/TLS certificates
  • Match hostname: Ensure certificate CN matches server hostname
  • Use correct port: Port 587 for STARTTLS, 465 for SSL/TLS
  • Update TLS version: Use TLS 1.2 or higher
  • For testing: Temporarily disable certificate verification (not for production)

Invalid Recipient / User Unknown

Error Code: 550, 550-5.1.1, User unknown

Possible Causes:

  • •Email address does not exist
  • •Typo in recipient email address
  • •Recipient's mailbox has been deleted
  • •Domain doesn't accept mail

Solutions:

  • Verify address: Double-check recipient email address for typos
  • Check domain: Verify the domain has valid MX records
  • Confirm mailbox: Ensure recipient mailbox still exists
  • Test with known address: Try sending to a verified email first
SMTP Troubleshooting Process
Systematic approach to diagnosing SMTP issues
1

Test Basic Connectivity

Verify you can reach the SMTP server:

telnet smtp.gmail.com 587
2

Verify DNS Configuration

Check MX records and DNS resolution:

nslookup -type=MX gmail.com
3

Test SSL/TLS Connection

Verify SSL/TLS handshake:

openssl s_client -starttls smtp -connect smtp.gmail.com:587
4

Check Authentication

Test SMTP authentication with your credentials using our online tool or command-line utilities.

5

Review Server Logs

Check SMTP server logs for detailed error messages and connection attempts. Look for patterns or recurring issues.

6

Test from Different Location

Try connecting from a different network or server to isolate whether the issue is network-related or configuration-related.

Provider-Specific Troubleshooting
Common issues with popular email providers

Gmail SMTP Issues

  • →"Less secure apps": Create an app-specific password instead
  • →2FA required: Enable 2FA and generate app password
  • →Captcha blocking: Sign in via browser first

Office 365 SMTP Issues

  • →Modern Auth: May need OAuth2 instead of password
  • →Username format: Always use full email address
  • →Security defaults: Check tenant security settings

Amazon SES Issues

  • →Sandbox mode: Verify recipient addresses first
  • →SMTP credentials: Different from AWS access keys
  • →Region-specific: Use correct regional endpoint

SendGrid Issues

  • →API key as password: Use API key, not account password
  • →Username: Always use "apikey" as username
  • →Sender verification: Verify sender identity first
Preventing SMTP Issues
Best practices to avoid common problems
  • Regular testing: Test SMTP configuration periodically to catch issues early
  • Monitor logs: Set up logging and monitoring for SMTP connections
  • Use TLS: Always use SSL/TLS or STARTTLS for secure connections
  • Document settings: Keep detailed documentation of SMTP configurations
  • Implement retries: Add retry logic with exponential backoff for transient failures
  • Set up alerts: Configure alerts for authentication failures and connection errors
  • Keep certificates updated: Renew SSL/TLS certificates before expiration

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Related SMTP Resources

SMTP Test Online →SMTP Checker →SMTP Test Tool →How to Test SMTP →SMTP Tester Home →