Should I retry a 550 5.1.1 error?

No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.

How do I fix SMTP 550 5.1.1?

Remove the address from the list. This is a permanent failure and retrying it counts against you.

Is 550 5.1.1 a hard bounce or a soft bounce?

550 5.1.1 is a hard bounce. The receiving side has said the address can't receive mail, so a retry won't deliver it. Do not retry. It will fail every time.

Will verifying my list prevent 550 5.1.1?

Yes. A mailbox check asks the recipient's server whether the address exists without sending anything, so addresses that would return this code are flagged before the send instead of bouncing against your domain.

Permanent failure — do not retry

Hard bounceThe address can't receive mail

SMTP 550 5.1.1

mailbox does not exist

SMTP 550 5.1.1 means the recipient's mail server accepted the connection but rejected the address: the mailbox does not exist on that domain. It is a permanent failure and a hard bounce. Do not retry it — remove the address, because repeated sends to non-existent mailboxes are what damages sender reputation.

Should you remove the address?

Remove the address from your list

Yes, remove it. A 5.1.1 is the clearest hard bounce there is: the receiving server has said the mailbox does not exist, and sending again only adds another hard bounce to your record.

Hard bounce. Do not retry. It will fail every time.

How it appears

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table

550 5.1.1 The email account that you tried to reach does not exist.

550 5.1.1 unknown or illegal alias

What causes it

The address was mistyped, at signup or when it was copied into your list.
The mailbox was deleted, usually because the person left the organisation.
The address never existed and was invented to get past a signup form.
The local part is right but the domain is a lookalike — a missing letter or a .co instead of .com.

How to fix it

In order — try the first one first.

Remove the address from the list. This is a permanent failure and retrying it counts against you.
Check for a typo in the domain before deleting — a near-miss on a well-known provider is often recoverable.
If a whole batch returns 550 5.1.1, the list is stale or was bought. Verify the rest before sending again.
Verify addresses before the send rather than discovering this from bounces, because the bounce is already recorded against your domain.

Who sends it

Almost every mail server uses it. Gmail, Microsoft 365 and most hosted mail systems return it at the RCPT TO stage, so the bounce arrives within seconds of the send.

Gmail
Reply text
The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces.
Gmail and Google Workspace return this for consumer and Workspace addresses alike.
Source: Google Workspace Admin Help: Gmail SMTP errors and codes
Microsoft 365
Provider's description
Bad destination mailbox address
Microsoft lists a mistyped address or an address that doesn't exist in the destination system as the causes. Microsoft 365 often uses 5.1.10 for the same situation.
Source: Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online
Yahoo
Provider's description
You should remove the email address from your mailing list.
Yahoo's guidance for its "Recipient does not exist" category. Yahoo uses a 554 reply for this; see the Yahoo 554 page.
Source: Yahoo Sender Hub: SMTP Error Codes

Common questions

What does SMTP 550 5.1.1 mean?: SMTP 550 5.1.1 means the recipient's mail server accepted the connection but rejected the address: the mailbox does not exist on that domain. It is a permanent failure and a hard bounce. Do not retry it — remove the address, because repeated sends to non-existent mailboxes are what damages sender reputation.
Should I retry a 550 5.1.1 error?: No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
How do I fix SMTP 550 5.1.1?: Remove the address from the list. This is a permanent failure and retrying it counts against you.
Is 550 5.1.1 a hard bounce or a soft bounce?: 550 5.1.1 is a hard bounce. The receiving side has said the address can't receive mail, so a retry won't deliver it. Do not retry. It will fail every time.
Should I remove the address after a 550 5.1.1 bounce?: Yes, remove it. A 5.1.1 is the clearest hard bounce there is: the receiving server has said the mailbox does not exist, and sending again only adds another hard bounce to your record.
Will verifying my list prevent 550 5.1.1?: Yes. A mailbox check asks the recipient's server whether the address exists without sending anything, so addresses that would return this code are flagged before the send instead of bouncing against your domain.

Clean the list before the next send.

A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.

A real SMTP mailbox check on every row, before anything is sent
Dead and mistyped addresses split out, so they never reach your sending tool
Catch-all, disposable and role addresses flagged alongside

Get started — $39/mo See the published limits first

Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.

Other 550 replies

Sources

Provider wording is quoted only where the provider publishes it.

Last reviewed 2026-10-22