Email Deliverability

HELO Hostname Mismatch

During an SMTP connection, the sending server introduces itself with a HELO or EHLO command followed by its hostname. Receiving servers verify that this hostname resolves to the connecting IP address. A mismatch between the HELO hostname, reverse DNS, and the actual server identity is a strong spam indicator.

Symptoms

  • Anti-spam filters flagging HELO hostname checks
  • Email headers showing HELO verification failures
  • Increased spam scoring on outbound emails
  • Some receiving servers rejecting connections at HELO stage

Common Causes

  • Mail server configured with a generic hostname (e.g., localhost, mail)
  • HELO hostname doesn't match the server's PTR record
  • Using the server's internal hostname instead of the public FQDN
  • Mail server migrated without updating HELO configuration

Step-by-Step Troubleshooting

  1. 1

    Check current HELO hostname

    Test SMTP connection to see what HELO hostname your server is sending.

    Use SMTP TLS Test →
  2. 2

    Verify reverse DNS

    Ensure PTR record matches the expected HELO hostname.

    Use Reverse DNS →
  3. 3

    Verify forward DNS

    Confirm the HELO hostname resolves to the server's IP.

    Use A Record Lookup →
  4. 4

    Update mail server config

    Set the HELO/EHLO hostname to match your server's FQDN and PTR record.

Recommended Diagnostic Tools

SMTP TLS TestReverse DNSA Record Lookup

Monitor SMTP Configuration

Track SMTP HELO hostname and TLS settings continuously.

Related Problems

Frequently Asked Questions