Free SMTP Server Checker (MX Record Validator)

Validate SMTP and MX configuration for your domain. Check mail server accessibility, STARTTLS support, and SMTP capabilities.

SMTP Server Checker

Validate SMTP/MX configuration and mail server accessibility

Enter your domain to check SMTP and MX configuration

What is SMTP and MX Record Validation?

Overview

SMTP (Simple Mail Transfer Protocol) is the protocol used to send and receive email between mail servers. MX (Mail Exchange) records in DNS tell other mail servers where to deliver email for your domain. Proper SMTP and MX configuration is fundamental to email deliverability - if your MX records are missing or your SMTP servers are inaccessible, you won't receive any email.

How MX Records Work

  1. Email Sent: Someone sends an email to user@yourdomain.com
  2. DNS Lookup: The sending server looks up MX records for yourdomain.com
  3. Priority Sorting: MX records are sorted by priority (lowest number = highest priority)
  4. Connection Attempt: The sending server attempts to connect to the highest priority MX server
  5. Fallback: If the first server fails, it tries the next priority MX record
  6. SMTP Handshake: Once connected, the SMTP conversation begins to deliver the email

MX Record Priority

Each MX record has a priority value (0-65535). Lower numbers indicate higher priority. This allows you to configure backup mail servers:

example.com. MX 10 mail1.example.com.
example.com. MX 20 mail2.example.com.
example.com. MX 30 mail3.example.com.

Sending servers will try mail1 first, then mail2 if mail1 is down, then mail3 as a last resort.

SMTP Server Capabilities

Modern SMTP servers advertise their capabilities during the SMTP handshake. Important capabilities include:

  • STARTTLS: Support for encrypting the connection with TLS
  • SIZE: Maximum message size the server will accept
  • 8BITMIME: Support for 8-bit characters (international characters)
  • PIPELINING: Allows multiple commands to be sent at once for faster delivery
  • AUTH: Authentication mechanisms supported (LOGIN, PLAIN, CRAM-MD5)
  • SMTPUTF8: Support for UTF-8 email addresses and headers

Why SMTP/MX Configuration Matters

  • Email Reception: Without valid MX records, you won't receive any email
  • Redundancy: Multiple MX records provide failover if your primary mail server goes down
  • Security: STARTTLS support encrypts email in transit
  • Deliverability: Properly configured SMTP servers improve sender reputation
  • Compliance: Some industries require encrypted email transport

Common SMTP/MX Issues

No MX Records

Domain has no MX records configured. Email delivery will fail or fall back to A record (not recommended). Add proper MX records pointing to your mail server.

SMTP Server Unreachable

MX records exist but the SMTP server isn't accepting connections. Check firewall rules, ensure port 25 is open, and verify the server is running.

No STARTTLS Support

SMTP server doesn't support TLS encryption. This is a security risk and may prevent email delivery from security-conscious senders. Enable STARTTLS with a valid certificate.

Invalid MX Hostname

MX record points to a hostname that doesn't exist or doesn't resolve to an IP address. Verify MX targets have valid A or AAAA records.

Single MX Record (No Redundancy)

Only one MX record means no failover if the server goes down. Consider adding a backup MX server with higher priority number.

Setting Up MX Records

Step-by-Step Configuration

  1. Choose Mail Server: Decide whether to host your own or use a provider (Google Workspace, Microsoft 365, etc.)
  2. Get MX Records: Obtain the MX record values from your email provider
  3. Access DNS Settings: Log into your domain registrar or DNS hosting provider
  4. Add MX Records: Create MX records with the hostname and priority values provided
  5. Remove Old Records: Delete any old or incorrect MX records
  6. Wait for Propagation: DNS changes can take 1-48 hours to propagate globally
  7. Test Configuration: Use this tool to verify MX records and SMTP connectivity
  8. Send Test Email: Send a test email to your domain to confirm delivery

MX Record Best Practices

  • Configure at least two MX records for redundancy (different priorities)
  • Ensure all MX hostnames have valid A or AAAA records
  • Use different servers or data centers for backup MX records if possible
  • Set reasonable TTL values (3600-86400 seconds) for MX records
  • Never point MX records to CNAME records (use A/AAAA only)
  • Ensure all MX servers support STARTTLS with valid certificates
  • Monitor MX server uptime and connectivity regularly
  • Keep MX priority numbers spaced apart (10, 20, 30) for flexibility

SMTP Ports and Protocols

Port 25 (SMTP)

Standard port for server-to-server email delivery. Must be open on receiving mail servers. Often blocked by ISPs for residential connections.

Port 587 (Submission)

Used for email clients to submit outgoing mail to a server. Requires authentication and uses STARTTLS for encryption.

Port 465 (SMTPS)

SMTP over implicit TLS. Connection is encrypted from the start. Commonly used by email clients as an alternative to port 587.