550 5.1.1 user unknown: why email bounces and what to do

The mailbox doesn't exist. How to read email error codes, what to check as the sender, and what to fix when real addresses on your domain bounce.

October 5, 2026 · 3 min read · By the Spot Downtime team

550 5.1.1 is the receiving mail server saying the mailbox you wrote to doesn't exist. It's a hard bounce: retrying won't help. The wording varies by provider:

text

550 5.1.1 The email account that you tried to reach does not exist.   (Gmail)
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup   (Microsoft 365)
550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table   (Postfix)

Reading the code

Email error codes have three parts. The first digit is the severity, and the rest narrows down the reason:

Code	Meaning	Retry?
`4.x.x`	Temporary failure: try again later	Yes, automatically
`5.1.1`	Mailbox doesn't exist	No
`5.1.2`	The domain part is wrong or has no mail server	No
`5.2.2`	Mailbox full	Later, maybe
`5.7.x`	Rejected by policy: authentication, spam or blocklists	Not until fixed

If you sent the email

Check the spelling, especially the part after the @. gmial.com and outlok.com are common.
The person may have left. Work addresses are removed when someone leaves a company.
The address may have changed after a company rename or a move to a new domain.

If you're the receiving domain

When real addresses on your domain bounce with 5.1.1, check what's answering for your mail:

MX records point to the wrong provider

After moving from one email provider to another, an old MX record left behind sends some mail to a server that no longer has your mailboxes. Only the new provider's MX records should remain.

MX record lookupSee which servers receive mail for a domain, in priority order, and which provider runs them.

The mailbox or alias doesn't exist yet

A new support@ or billing@ address that was printed on a website but never created, or an alias removed during a cleanup.

The domain isn't verified with the provider

Microsoft 365 and Google Workspace reject mail for a domain that was removed or never fully verified in the admin console.

For anyone sending bulk email

Remove hard-bounced addresses from your lists after the first 5.1.1. Mailbox providers watch your bounce rate, and repeatedly sending to addresses that don't exist hurts delivery for all your other mail.

If your bounces say 5.7.x instead, the address exists and you were refused on policy: see 550 5.7.26 and 554 5.7.1.