550 5.1.1 is the most common hard bounce there is. It means the receiving server looked up the address you wrote to and found no mailbox with that name. The message was not delivered, and sending it again will not change anything.

This page explains what the code means, the versions you will see from Gmail and Microsoft, why it happens, and how to stop it before it hurts your sender reputation. For every other code, see our guide to SMTP bounce codes.

What the error looks like

The wording changes from one provider to another, but the code is the same:

Provider Typical message
Gmail and Google Workspace 550 5.1.1 The email account that you tried to reach does not exist.
Microsoft 365 / Outlook 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
Other servers 550 5.1.1 <name@example.com>: Recipient address rejected: User unknown

Microsoft uses 5.1.10 for the same situation: the address is not in its directory. Read it exactly like 5.1.1.

What it means

The code has two parts:

  • 550: a permanent failure. The receiving server refused the message for good.
  • 5.1.1: "bad destination mailbox address". The .1 in the middle means the problem is the address itself, and the last .1 means the mailbox part (the name before the @) does not exist on that domain.

So the domain is real and its mail server answered, but nobody by that name has a mailbox there. It is a hard bounce.

Why it happens

  • A typo when the address was collected: jonh@, john.smiht@, a missing letter on a form.
  • The person left the company. Business addresses are closed when someone leaves. B2B lists lose a share of their addresses every month this way.
  • An old or bought list. The older the list, the more mailboxes have been closed since.
  • A guessed address. firstname.lastname@ patterns are often wrong for a given company.
  • A fake address typed on purpose to get a download or a discount without giving a real email.

What to do when you get it

  1. Remove the address from your list now, and add it to your suppression list so a future import cannot bring it back.
  2. Do not resend. Each new attempt is another bounce counted against you.
  3. Look for a typo you can fix. If the address is jhon.doe@company.com, the right one may be john.doe@company.com. Only use a corrected address if you can confirm it.
  4. Check where it came from. If many 5.1.1 bounces come from the same form or the same list, fix the source.

If you are the person who mistyped the address, check the spelling and the domain, and ask the recipient for their current address another way.

Why it matters for your reputation

Mailbox providers watch how often you write to addresses that do not exist. A sender who keeps doing it looks like someone using an old, bought or scraped list, and their mail gets filtered more strictly, including the messages to real people. Sending platforms enforce limits too: Amazon SES, for example, reviews accounts at a 5% bounce rate. Aim to keep hard bounces under 2% of every send (see what a good bounce rate is).

How to prevent it before you send

A 550 5.1.1 can almost always be caught before you send, without sending anything:

  • Verify the list. An email verification service asks the receiving server whether the mailbox exists, the same way a real delivery starts, and stops before any message is sent. Addresses that would bounce with 5.1.1 come back as invalid. You can try one address with the free email checker, or check a whole list in MailQuell.
  • Verify again before reusing an old list, since mailboxes close over time.
  • Catch typos on your forms: a "did you mean gmail.com?" hint and a check at sign-up stop bad addresses from entering your list.

One limit to know: some domains are catch-all and accept every address during the check, then bounce later. Verification marks those as catch-all rather than safe; see catch-all emails.

Frequently asked questions

Is 550 5.1.1 a hard bounce?

Yes. It is a permanent failure caused by the address itself. Remove the address.

Can 550 5.1.1 be wrong?

Rarely, but it happens: a server that is being migrated, or a mailbox created minutes after your send. If an address that matters to you bounced, confirm it with the person another way before writing again.

Why did my message bounce with 5.1.1 hours after sending?

Some servers accept every message first and check the address later, then send the bounce back. This is typical of catch-all domains and some gateways.

Is 550 5.1.1 my server's fault?

No. Your server delivered the message to the right place; the mailbox simply does not exist there. A refusal aimed at you (your IP, your authentication) uses a 5.7.x code instead, such as 550 5.7.1 or 550 5.7.26.

Sources