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.
What Does Error 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.
Common Causes
- 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 Error 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
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.
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.
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.