X.1.7 · Addressing
SMTP enhanced status code 5.1.7
Bad sender's mailbox address syntax
X.1.7 is about your side: the sender address in MAIL FROM or the headers is not valid syntax. The receiving server refused the message because it could not parse the address you are sending from.
| Registry code | X.1.7 |
|---|---|
| Forms in use | 5.1.7, 4.1.7 |
| Registry name | Bad sender's mailbox address syntax |
| Subject | 1: Addressing |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | Not given |
| Defined in | [RFC 3463] |
What the registry says
The sender's address was syntactically invalid. This can apply to any field in the address.
IANA SMTP Enhanced Status Codes registry, X.1.7
In practice
X.1.7 is triggered by your own sender address: an envelope sender with a space or unbalanced quotes, a From header with a display name that breaks the syntax, or a sending tool that inserts an empty or malformed MAIL FROM. It affects every recipient, not one, which is the clearest way to tell it apart from recipient errors. Send a test to an address you control and look at the raw headers.
What the sender should do
Check the envelope sender and From address your sending tool uses, including display-name quoting and stray characters. Verifying the recipient list will not change this.
Would verifying the list have prevented 5.1.7?
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.
How providers use X.1.7
Quoted from each provider’s own error reference.
- Gmail 553 5.1.7
The sender address address is not a valid RFC 5321 address. For more information, go to About SMTP error messages and review RFC 5321 specifications.
Common questions
- What does 5.1.7 mean?
- X.1.7 is about your side: the sender address in MAIL FROM or the headers is not valid syntax. The receiving server refused the message because it could not parse the address you are sending from.
- What is the difference between 5.1.7 and 4.1.7?
- They share the subject and detail digits, so they report the same condition, "Bad sender's mailbox address syntax". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.1.7?
- Check the envelope sender and From address your sending tool uses, including display-name quoting and stray characters. Verifying the recipient list will not change this.
- Would an email verifier have caught 5.1.7?
- 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
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and 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