Short answer
550 5.1.1 means the receiving server doesn't have a mailbox for that address. RFC 3463 calls X.1.1 "bad destination mailbox address": the domain is right, the part before the @ isn't. Gmail's version reads "The email account that you tried to reach does not exist".
This is the classic hard bounce. Don't retry; remove or suppress the address, and if it came from a sign-up form, ask the user to correct it.
On this page
What 550 5.1.1 means
The basic code 550 is "mailbox unavailable". The enhanced code 5.1.1 sits in the addressing subject (.1) and says the destination mailbox address is bad. The domain accepted the connection, so its mail server exists; it just has no such user.
Close relatives: 5.1.10 (Microsoft 365: "Recipient not found" by SMTP address lookup), 5.1.2 (bad destination system, usually a domain that doesn't exist) and 5.2.1 (mailbox disabled or inactive).
Common causes, by receiver
| Receiver | Typical text | Notes |
|---|---|---|
| Gmail | "The email account that you tried to reach does not exist" | Google suggests checking for typos and extra spaces. |
| Microsoft 365 | "Bad destination mailbox address" or 550 5.1.10 "Recipient not found" | A mistyped address, or a mailbox that was removed or renamed. |
| Outlook.com / Hotmail | 550 5.5.0 Requested action not taken: mailbox unavailable | Microsoft documents this as the consumer equivalent of 5.1.10. |
| Yahoo | "This user doesn't have a yahoo.com account" (often with 554) | Yahoo says not to retry and to remove the address. |
From Google's Gmail SMTP error reference, Microsoft's NDR reference and Yahoo's SMTP error codes.
- Typos at sign-up:
gmial.comusually fails earlier, butjon.smith@instead ofjohn.smith@lands here. - Old addresses: people leave jobs and close accounts. A list that hasn't been mailed in a year will have some.
- Imported or bought lists: a high share of 5.1.1 bounces is a sign the list wasn't collected with consent.
How to fix it as a sender
- Stop sending to the address. Repeatedly mailing unknown users is one of the clearest signals of a poor list, and mailbox providers weigh bounce rates when they score you.
- Fix it at the source. Validate addresses at sign-up, confirm with a verification email, and offer a way for users to correct a mistyped address.
- Check the domain if many bounce at once. If every address at one domain bounces with 5.1.1, the domain may have moved its mail elsewhere; check its records with the MX lookup.
- Keep your bounce rate low. Mailbox providers notice senders who keep mailing unknown users. Aim well under 2%.
Retry or suppress?
How Groundmail handles 550 5.1.1
When a receiver answers a Groundmail send with 550 5.1.1 because the mailbox doesn't exist, the email is marked bounced with bounce_type: "hard", the receiver's reply is kept in bounce_reason, an email.bounced webhook fires, and the address goes on your suppression list automatically. Later sends to it return 422 with the address listed under suppressed instead of bouncing again, which protects your bounce rate.
Before a send goes out, Groundmail validation also holds back addresses that can't receive mail (a domain with no mail server, or a mailbox already known not to exist), so many of these never bounce at all. See suppressions and emails in the docs.
Frequently asked questions
Is 550 5.1.1 a hard bounce?
Yes. It's permanent and it's about the address: the mailbox doesn't exist. Suppress the address rather than retrying.
The address worked last month. Why does it bounce now?
Mailboxes get deleted or renamed, especially work addresses when someone leaves. Once the receiver says 5.1.1, treat the address as gone until the person gives you a new one.
Can a 550 5.1.1 be wrong?
Rarely, a misconfigured receiving server rejects real users, for example during a mail migration. If many valid addresses at one domain bounce at once, contact that domain's admin. For a single address, trust the bounce.
What's the difference between a soft bounce and a hard bounce?
A soft bounce is a temporary failure, usually a 4xx reply (mailbox busy, rate limited, try again later); the sending server retries it. A hard bounce is a permanent failure, a 5xx reply, where retrying the same message won't help. Among hard bounces, it matters whether the problem is the address (unknown user: suppress it) or the sender (a block: fix the cause, keep the address).
Sources
Facts on this page about other companies' products, prices and policies come from these public pages. They change, so check them before you decide.