550 5.1.1 📧 Email / SMTP

550 5.1.1 The email account that you tried to reach does not exist (User unknown)

The recipient mailbox doesn’t exist on the receiving server — a typo in the address, a deleted account, or a wrong domain.

Seen on: SMTP

Meaning

5.1.1 is a permanent failure (hard bounce). Retrying won’t help. Mailing lists should remove the address to protect sender reputation.

Common causes

  • Typo in the email address
  • Mailbox deleted or never existed
  • Wrong domain (gmial.com)
  • Recipient server misconfigured (catch-all expected)

⚡ Quick fix

  1. Check the spelling and domain
  2. Ask the recipient for a correct address
  3. Remove hard-bounced addresses from lists

Detailed fix by platform

Shell

  1. dig +short MX example.com # domain exists and accepts mail?

How to diagnose

  1. Address — Exact spelling
  2. Domain — MX records exist?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

🧠 Still stuck? Analyze your error

Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.