EC
5.1.1
SMTPERRORNotableAddressingHIGH confidence

Bad destination mailbox address — mailbox does not exist

Production Risk

Low for individual messages; High for sender reputation if not handled (hard bounces spike).

What this means

The destination mailbox does not exist on the receiving mail system. This is the most common cause of email bounces (hard bounce).

Why it happens
  1. 1The recipient's email address was mistyped.
  2. 2The recipient's account has been deleted.
  3. 3The domain exists but the specific mailbox does not.
How to reproduce

Sending mail to a non-existent address. Common in bulk mailers with stale lists.

trigger — this will error
trigger — this will error
RCPT TO:<nonexistent@example.com>
# Server returns:
550 5.1.1 The email account that you tried to reach does not exist

expected output

550 5.1.1 ...

Fix

Remove invalid address from mailing list

WHEN Received as a bounce in bulk sending

Remove invalid address from mailing list
# Mark address as hard-bounced in your database
UPDATE subscribers SET status = 'hard_bounce' WHERE email = 'nonexistent@example.com';
# Never retry hard bounces

Why this works

Hard bounces (5.x.x) should never be retried — remove them immediately from your list.

What not to do

✕ Retry a 5.1.1 bounce

5.x.x codes are permanent failures. Retrying will damage your sender reputation.

Sources
Official documentation ↗

RFC 3463 — Enhanced Mail System Status Codes

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