Enhanced Status Code 5.1.7: Bad Sender Mailbox Address Syntax

Permanent failure High severity Address RFC 3463

What it means

Enhanced Status Code 5.1.7 means “Bad Sender Mailbox Address Syntax.” Your sender (From) email address has a syntax error. The receiving server rejected the message because the MAIL FROM address is malformed, contains invalid characters, or does not conform to email address standards.

At a glance

Code5.1.7

Bounce typeHard (permanent)

SeverityHigh

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

host mail.example.com[198.51.100.10] said: 501 5.1.7 Bad sender address syntax (in reply to MAIL FROM command)

What does 5.1.7 mean?

Enhanced status code 5.1.7 means the receiving server found a syntax problem with your sender address (the MAIL FROM / Return-Path address). This is a sender-side configuration issue. The address used in the SMTP envelope or the From header is malformed.

This is different from recipient address errors (5.1.1, 5.1.3) because the problem is with YOUR address, not the recipient's. Common causes include misconfigured return-path addresses, blank or null sender addresses where one is required, or addresses with invalid characters.

How 5.1.7 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.7 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 5.1.7 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
5.1.7 is		✓ this code

Common causes of 5.1.7

MAIL FROM address has invalid syntax or characters
Return-Path header is malformed
Sender address is blank or null where not permitted
From header address does not comply with RFC 5322

How to fix 5.1.7

Verify your MAIL FROM / Return-Path address is correctly formatted
Check your MTA configuration for the sender address setting
Ensure the From header in your emails uses a valid address format
Test your SMTP conversation to see exactly what sender address is being sent

Frequently asked questions

What does bounce code 5.1.7 "Bad sender's mailbox address syntax" mean?

Bounce code 5.1.7 means the sender's email address (in the MAIL FROM field) has a syntax error and does not conform to email formatting standards. The receiving server rejected the message because it could not validate the sender address. Common issues include a missing "@" symbol, prohibited characters, incorrect dot placement, or a domain name that cannot be resolved by DNS.

How do I fix SMTP error 501 5.1.7?

Verify your sender email address is properly formatted as "user@domain.com" with no extra spaces, special characters, or missing components. Check that your sending domain exists and has valid DNS records. If using Postfix with "strict_rfc821_envelopes" enabled, ensure email addresses are enclosed in angle brackets (<user@domain.com>). Review your email client or application's SMTP configuration to confirm the sender address is correctly entered.

Why does my printer or application get a 5.1.7 error when sending email?

Printers, scanners, and applications often trigger 5.1.7 errors because they are configured with improperly formatted sender addresses. Many devices use simplified email fields that omit required formatting such as angle brackets or proper domain names. Configure the device's "From" address as a complete, valid email address (user@yourdomain.com), ensure the domain has valid MX and A records, and check that the SMTP server accepts the authentication method your device supports.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes