Whylidate
Permanent FailureHigh Severity

SMTP Error 553 5.1.3

Invalid Address Format

The email address syntax is invalid.

What This Error Means

The email address does not conform to standard email address format (RFC 5321). This is a permanent error indicating the address cannot receive mail.

Common Causes

  • 1Missing @ symbol
  • 2Invalid characters in local part
  • 3Domain portion is malformed
  • 4Spaces or special characters not properly encoded

How to Fix It

  • 1Validate email format before adding to list
  • 2Use regex or library validation
  • 3Implement real-time validation on signup forms
  • 4Remove malformed addresses from your list

Prevention with Whylidate

Prevent this error before it happens by validating email addresses with Whylidate. Our real-time verification catches invalid addresses, full mailboxes, and policy issues before you send.

Quick Reference

Error Code
553 5.1.3
Category
Permanent Failure
Severity
High
Should Retry?
No

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