Gmail 501 5.5.4
Invalid HELO/EHLO argument
Gmail 501 5.5.4 means: invalid HELO/EHLO argument. It is a permanent failure, so do not retry it unchanged. Send a valid fully qualified hostname in HELO or EHLO, not an empty value, an IP without brackets or someone else's hostname.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
HELO/EHLO argument argument invalid closing connection. For more information, go to HELO/EHLO email error.
Reply text
Empty HELO/EHLO argument not allowed, closing connection. For more information, go to HELO/EHLO email error.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 501: Syntax error in parameters or arguments (RFC) |
| Enhanced code | 5.5.4: Invalid command arguments |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
Gmail rejects an empty HELO and an invalid argument. The name should be your server's fully qualified domain name, ideally matching its PTR record. Scripts often send localhost, a bare IP address without brackets, or nothing at all. Fixing it is a one-line change in the client configuration. Many receivers apply the same check, not only Gmail.
What the sender should do
Send a valid fully qualified hostname in HELO or EHLO, not an empty value, an IP without brackets or someone else's hostname.
Would verifying the list have prevented Gmail 501 5.5.4?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Gmail 501 5.5.4 mean?
- Gmail 501 5.5.4 means: invalid HELO/EHLO argument. It is a permanent failure, so do not retry it unchanged. Send a valid fully qualified hostname in HELO or EHLO, not an empty value, an IP without brackets or someone else's hostname.
- Should I retry after Gmail 501 5.5.4?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Send a valid fully qualified hostname in HELO or EHLO, not an empty value, an IP without brackets or someone else's hostname.
- Would verifying my list have prevented Gmail 501 5.5.4?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05