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.

Permanent failure — do not retry

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.

How it appears

550 5.1.1 <user@example.com>: 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.

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.

Last reviewed 2026-09-11