Enhanced Status Code 5.1.1: Bad Destination Mailbox Address - User Unknown

Permanent failure Critical severity Address RFC 3463

What it means

Enhanced Status Code 5.1.1 means “Bad Destination Mailbox Address - User Unknown.” The recipient email address does not exist on the destination server. The specific user mailbox is invalid, deleted, or was never created. This is the single most common bounce code and the address must be removed from your list immediately.

At a glance

Code5.1.1

Bounce typeHard (permanent)

SeverityCritical

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

550-5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces.

What does 5.1.1 mean?

Enhanced status code 5.1.1 is the most common bounce code in email delivery. It definitively states that the recipient mailbox does not exist on the receiving server. The domain is valid and the server is reachable, but there is no mailbox for the specific user part of the address. This is a permanent failure - the address will never accept email until someone creates that mailbox.

Gmail returns "550-5.1.1 The email account that you tried to reach does not exist." Microsoft returns "550 5.1.1 The email account that you tried to reach doesn't exist." Yahoo returns "554 delivery error: dd This user doesn't have a yahoo.com account." Regardless of the provider, the meaning is the same: the address is invalid.

This code has the highest impact on sender reputation among all bounce codes. Mailbox providers interpret repeated sends to non-existent addresses as a sign of purchased lists, scraped addresses, or poor list management. Keep your 5.1.1 bounce rate as close to zero as possible by using email verification at signup and regularly cleaning your list.

How 5.1.1 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.1 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 5.1.1 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
5.1.1 is		✓ this code

Common causes of 5.1.1

Recipient email address was mistyped or has a typo
User account has been deleted or deactivated
Employee left the organization and their mailbox was removed
Email address was never valid (purchased or scraped list)
Domain catch-all was disabled, exposing invalid addresses

How to fix 5.1.1

Remove the address from your list immediately - never retry a 5.1.1
Check for common typos: gmial.com, outlok.com, yaho.com, etc.
Implement real-time email verification at signup forms
Use double opt-in to confirm all new subscriber addresses
Run your entire list through an email verification service
Set up automated suppression rules to remove 5.1.1 bounces instantly

Frequently asked questions

What does bounce code 550 5.1.1 mean?

Bounce code 550 5.1.1 means the recipient email address does not exist on the destination mail server. The server could not find a mailbox matching the address you sent to. This is a permanent (hard) bounce; the most common causes are a typo in the email address, a deleted account, or a deactivated mailbox. You should immediately remove or correct the address rather than retrying.

Is 5.1.1 a hard bounce or a soft bounce?

Error 5.1.1 is a hard bounce. The leading "5" in the status code indicates a permanent failure, meaning the recipient mailbox is invalid and resending the same message will not succeed. Email service providers like Gmail, Microsoft 365, and Yahoo treat 5.1.1 as a definitive rejection. You should suppress this address from your mailing list to protect your sender reputation.

How do I fix the 550 5.1.1 "User Unknown" error?

First, double-check the recipient email address for typos; a single misplaced character triggers this error. If the address appears correct, the account may have been deleted or deactivated by the recipient's organization. Contact the recipient through an alternative channel to confirm their current email address. On Microsoft Exchange servers, also verify that MX records point to the correct mail server and that recipient filtering is properly configured.

Why does Gmail return 550 5.1.1 when the user exists?

Gmail may return 5.1.1 even for existing accounts if the mailbox has been disabled or suspended due to policy violations, inactivity, or exceeding receiving rate limits. Additionally, Google Workspace admins may have configured the account to reject external mail. Check with the recipient's IT administrator to confirm the account status and ensure it is configured to receive external messages.

Should I remove 5.1.1 bounced addresses from my email list?

Yes, you should immediately and permanently suppress any address that returns a 5.1.1 bounce. Continuing to send to invalid addresses increases your bounce rate, damages your sender reputation with providers like Gmail and Microsoft, and can lead to your domain or IP being blocklisted. Most email service providers automatically suppress hard-bounced addresses after the first occurrence.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code Google Workspace, Fix bounced or rejected emails · Gmail help describing the "recipient's email address doesn't exist" bounce and what to check. Microsoft Learn, Fix NDR error 550 5.1.1 through 5.1.20 in Exchange Online · Microsoft guidance for the 5.1.x family where the recipient does not exist.

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes