550 5.1.1: Recipient Address Rejected, User Unknown

A 550 5.1.1 bounce means the receiving server looked up the mailbox you sent to and it doesn't exist. Either the address has a typo, or it was real once and has been deleted. It's a permanent failure, so resending won't help: fix the address if it's a typo, and remove it from your list if it isn't. One of these is a nuisance. A pile of them from a list send means the list has a quality problem.

What the bounce says

The wording varies by provider, but the code is the same. Gmail's version looks like this:

550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1  https://support.google.com/mail/?p=NoSuchUser

Other servers say it more tersely: "User unknown," "Recipient address rejected," "No such user," or "Mailbox not found." Microsoft 365 often uses a related code, 550 5.1.10, for the same situation (Microsoft's docs describe it as the recipient's address not existing or not being found).

What it means

The code has two parts. 550 is the basic SMTP reply: the server won't accept the message for this recipient. 5.1.1 is the enhanced status code from RFC 3463. The 5 means permanent failure, the first 1 means the problem is with an address, and the second 1 means "bad destination mailbox address," which the RFC defines as "The mailbox specified in the address does not exist." The domain is fine; the part before the @ isn't.

Some related codes are easy to confuse with it:

A 5.1.1 is not a reputation block and has nothing to do with spam filtering. If your bounce says something about spam, policy or a blocklist, you're looking at a different problem. See the Gmail 550 5.7.1 guide or the blocklist playbook.

Common causes, most likely first

1. A typo. jsmith@exmaple.com, a missing letter, an extra space, or a period in the wrong place. On sign-up forms, this is the biggest single source.
2. The account was deleted. Someone left the company, or a personal account was abandoned and closed. Google, for example, may delete personal accounts after two years of inactivity.
3. An old list. Addresses collected years ago, imported from a CRM, a trade show or a previous ESP, and never mailed since. People change jobs and abandon accounts, so an unmailed list decays every month it sits.
4. Fake or bot sign-ups. Made-up addresses entered to get a discount code, or bots submitting junk through your forms.
5. The recipient's mail routing is wrong. If a company moved its email to a new provider but its MX records (the DNS records that say where its mail goes) still point at the old one, the old provider rejects mail for users it no longer has. The address is real, just not where the internet thinks it is.

How to fix it

If you're sending one-to-one:

1. Read the address in the bounce carefully. Compare it character by character with what you meant to type.
2. Clear the autocomplete entry. Microsoft's guidance is to delete stale suggestions from Outlook's autocomplete list and retype the address in full.
3. Ask the recipient another way. A phone call or a message on another channel will tell you whether the address changed.
4. If the address is definitely right, the recipient's side may be misrouted. Let them know; their admin will need to check their MX records or forwarding rules.

If you're sending to a list:

1. Make sure hard bounces are suppressed. Most ESPs suppress 5.1.1 addresses automatically (SendGrid, for one, keeps them on a bounce suppression list). Don't override it, and don't re-import those addresses.
2. Look at where the bounces came from. Sort bounced addresses by sign-up source and date. If one form, one import or one old segment accounts for most of them, that's your problem.
3. Fix the source. Add typo checking and confirmed (double) opt-in to forms that attract bad addresses, and bot protection to anything that sends an automatic email. Yahoo's sender guidance recommends double opt-in for exactly this reason.
4. Retire old data carefully. Before mailing a list you haven't touched in a long time, validate it and send to the most recently active part first.

How to confirm it's fixed

For a single address: resend after correcting it. If it delivers, you're done.

For a list: watch your bounce rate per send over the next few campaigns. It should drop sharply once the bad addresses are suppressed and the source is fixed. If 5.1.1 bounces keep showing up at the same rate, new bad addresses are still coming in somewhere.

How to keep it from coming back

• Use confirmed opt-in on public forms, or at least validate the address at the point of entry.
• Keep mailing your list regularly. Addresses that get mail every month are noticed when they die; addresses mailed once a year come back as a surprise pile of bounces.
• Remove invalid recipients promptly, as Yahoo's sender guidance asks.
• Watch for sudden sign-up spikes from one source, which often means bots.

If bounces are part of a bigger deliverability drop, the ESP guides for Klaviyo, Mailchimp and SendGrid cover the wider picture.

When to get help

If your hard bounce rate is high across many campaigns and you can't find the source, or the bounces come with a drop in inbox placement, it's worth having someone look at your list sources and sending data. Get in touch.

FAQ

Is a 550 5.1.1 bounce my fault?

Usually it means the address was mistyped or no longer exists, not that you did anything wrong. It only becomes your problem when you keep sending to addresses that bounce, or when a lot of your list bounces at once.

Should I retry a 550 5.1.1?

No. It's a permanent failure. Retrying sends the same message to the same nonexistent mailbox. Fix the address or remove it.

Do 5.1.1 bounces hurt my sender reputation?

A few don't matter. A lot of them do, because a high hard bounce rate suggests a stale or purchased list, and mailbox providers weigh that when deciding whether your mail is wanted.