SMTP Error 550: Requested Action Not Taken - Mailbox Unavailable

Permanent failure Critical severity Address RFC 5321

What it means

SMTP Error 550 means “Requested Action Not Taken - Mailbox Unavailable.” The recipient email address is invalid, does not exist, or has been blocked. This is the most common permanent bounce code and a strong signal that the address should be removed from your list immediately. Continued sending to 550 addresses damages sender reputation.

At a glance

Code550

Bounce typeHard (permanent)

SeverityCritical

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 5321

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 550 mean?

SMTP code 550 is the most common and most important hard bounce code in email delivery. It means the receiving server has permanently rejected your email because the requested mailbox is unavailable. The most frequent cause is an invalid or non-existent email address - the user does not exist on that server.

However, 550 is also used for policy-based rejections. A 550 response may include enhanced status codes like 5.1.1 (user unknown), 5.7.1 (policy rejection), or 5.7.26 (DMARC failure) that provide more specific information. Always check the enhanced code and the human-readable text that accompanies the 550 response to understand the exact reason.

Gmail returns 550 5.1.1 for non-existent addresses, 550 5.7.1 for messages blocked due to content or reputation, and 550 5.7.26 for DMARC failures. Microsoft returns 550 5.1.1 for invalid users and 550 5.7.1 for policy blocks. Yahoo uses 550 with various sub-codes for address and policy failures. Every 550 bounce to an invalid address damages your sender reputation. Keep your bounce rate below 2% by maintaining clean lists.

How 550 plays out

Your server attempts delivery

The recipient server returns a permanent 550 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 550 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
550 is		✓ this code

Common causes of 550

Recipient email address does not exist (typo, deleted account)
Mailbox has been deactivated or suspended
Domain exists but the specific user account is invalid
Sender IP or domain is blocked by the recipient server policy
DMARC, SPF, or DKIM authentication failure causing rejection
Content or reputation-based policy rejection

How to fix 550

Remove invalid addresses from your mailing list immediately
Check the enhanced status code (5.1.1, 5.7.1, etc.) for specific cause
If policy-based (5.7.1), check your sender reputation, authentication, and blacklist status
Implement email verification at signup to prevent invalid addresses from entering your list
Use double opt-in to confirm all new subscriber addresses
Run your list through an email verification service before sending campaigns

Frequently asked questions

What does SMTP error 550 mean?

SMTP error 550 means the recipient's mail server has permanently rejected your email and will not attempt redelivery. This is classified as a "hard bounce" and typically occurs because the recipient address does not exist, the recipient's mailbox is full or disabled, or the receiving server has blocked your message due to spam filters or policy restrictions. Common sub-codes include 550 5.1.1 (user unknown), 550 5.4.1 (access denied), and 550 5.7.1 (security/policy violation).

Is SMTP error 550 a hard bounce or soft bounce?

SMTP error 550 is a hard bounce, meaning it represents a permanent delivery failure. Because 550 is a 5xx-class status code, the sending server will not automatically retry delivery. You must resolve the underlying issue, such as correcting the recipient address, removing the address from your list, or fixing your SPF/DKIM/DMARC authentication records, before the email can be successfully delivered.

How do I fix a 550 email error in Gmail or Outlook?

To fix a 550 error, first verify the recipient's email address for typos or formatting errors. Then check that your domain has valid SPF, DKIM, and DMARC records configured using a tool like MXToolbox. For Gmail, ensure you have not exceeded the daily sending limit of 500 emails (or 2,000 for Google Workspace). For Microsoft 365/Outlook, verify that your sending IP is not on a blocklist and that SMTP authentication is enabled in your outgoing server settings.

Why am I getting "550 permanent failure for one or more recipients"?

The "550 permanent failure for one or more recipients" error means at least one email address on your message is invalid, disabled, or actively rejecting your mail. Common causes include sending to a non-existent mailbox, a full recipient inbox, your domain or IP being listed on a blacklist (such as Spamhaus or Barracuda), or missing email authentication records. Immediately remove invalid addresses from your list and use an email verification service to clean your contacts.

Can a 550 error be caused by my IP being blacklisted?

Yes, a 550 error is frequently caused by your sending IP or domain being on a DNS-based blocklist (DNSBL). Services like Spamhaus, UCEPROTECT, and Barracuda maintain blacklists that receiving servers check when processing inbound mail. You can verify your blacklist status using MXToolbox's Blacklist Check tool, and if listed, follow each blacklist provider's specific delisting process to restore your sending reputation.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code Google: Why your email bounced · Gmail SMTP error reference, including 550 5.1.1, 5.7.1 and 5.7.26 Microsoft: Email non-delivery reports in Exchange Online · Maps NDR codes such as 550 5.1.1 and 550 5.7.1 to causes and fixes

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