SMTP code 501 5.5.4
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
The knowledge base documents this code in more than one row. What tells those rows apart is the rejection text in your own log, not the code: read each line below against what your rejection says.
- Test that tells causes apart
- Verify that a PTR record exists and matches, and that HELO gives a fully qualified domain name (FQDN), not a private IP address
- Next step
- Fix rDNS and HELO. Receivers, including Microsoft, reject mail that advertises 10.x, 172.16–31.x or 192.168.x
- Rejection says…
- HELO or EHLO error, 501 5.5.4, "invalid HELO", nonroutable IP address
- Hypothesis
- Connection identity or reverse DNS (rDNS)
From Delivery Troubleshooting Playbooks · Playbook A: message not delivered (bounced or rejected, 5xx)
This line comes from a knowledge base article on Gmail, not on the standard.
- Message
- "HELO/EHLO argument invalid" / "Empty HELO/EHLO argument not allowed"
- Meaning and fix
- Send a valid fully qualified domain name (FQDN) in HELO or EHLO
From Gmail SMTP Errors and Troubleshooting · Protocol and session