553
SMTP Error 553: Mailbox Name Not Allowed
Permanent failure
High severity
Address
RFC 5321
What it means
SMTP Error 553 means “Mailbox Name Not Allowed.” The recipient email address has a syntax problem, uses characters not allowed by the server, or the server policy does not allow that particular mailbox name. This can also indicate a relay attempt that the server refuses to process.
At a glance
Code553
Bounce typeHard (permanent)
SeverityHigh
CategoryAddress
What to doSuppress the address; do not retry
StandardRFC 5321
553 5.7.1 <user@example.com>: Sender address rejected: not owned by user authuser@example.comWhat does 553 mean?
SMTP code 553 indicates the mailbox name provided in the RCPT TO command is not allowed by the receiving server. This can mean the address format is invalid, contains prohibited characters, or the server policy restricts certain mailbox names. It differs from 550 (user unknown) because the issue is with the address format or policy rather than the user not existing.
Some servers return 553 when they detect what they consider a relay attempt - when the sender tries to use the server to deliver mail to a domain the server is not responsible for. This is a security measure to prevent unauthorized relay abuse.
How 553 plays out
Your server attempts delivery
The recipient server returns a permanent
553 rejectionThis is a hard bounce: the message will not be accepted as sent
Suppress the address and fix the root cause before resending
Where 553 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 |
| 553 is | ✓ this code |
Common causes of 553
- Email address contains invalid or prohibited characters
- Address format does not match server requirements
- Attempting to relay through a server that does not allow open relay
- Server policy restricts certain mailbox name patterns
- Catch-all or wildcard address not accepted
How to fix 553
- Verify the recipient email address format is correct
- Remove special characters that may not be allowed
- Ensure you are connecting to the correct mail server for the recipient domain
- Check that you are not attempting to relay through an unauthorized server
Frequently asked questions
What does SMTP error 553 mean?
SMTP error 553 means "Requested action not taken: mailbox name not allowed," indicating that the email address used in the "From" field is invalid, improperly formatted, or not authorized by the sending or receiving server. This is a permanent failure most commonly caused by missing SMTP authentication, a mismatch between your email domain and the server you are sending through, or an incorrectly formatted sender address.
How do I fix SMTP error 553 in Outlook?
To fix error 553 in Outlook, enable SMTP authentication in your outgoing server settings. Go to File > Account Settings, select your email account, click "Change," then "More Settings," and navigate to the "Outgoing Server" tab. Check the box for "My outgoing server (SMTP) requires authentication" and select "Use same settings as my incoming mail server." Also verify that the email address in your "From" field matches the account you are authenticated with, as relay restrictions will block mismatched sender addresses.
What does "553 relay not permitted" mean?
"553 relay not permitted" means the mail server is refusing to forward your email because you have not authenticated, or you are trying to send through a server that does not authorize your domain for relaying. Most SMTP servers require a valid login before they will relay messages to external recipients. To fix this, enable SMTP authentication in your email client, verify your username and password are correct, and ensure the outgoing SMTP server matches your email provider's recommended settings.
Why does error 553 say my mailbox name is not allowed?
The "mailbox name not allowed" message in a 553 error typically means the "From" address in your email is either malformed (e.g., missing the @ symbol, contains invalid characters), does not exist on the sending server, or the sending domain's DNS records (especially SPF) do not authorize the server you are using. This can also occur if the recipient's domain is non-existent or has expired DNS records. Double-check both sender and recipient addresses and verify your domain's SPF record includes your sending server's IP address.
Is SMTP error 553 the same as error 550?
No, while both are permanent 5xx errors, they have different causes. Error 550 typically means the recipient's mailbox is unavailable or the message was rejected by the recipient's server due to policy, spam filtering, or an invalid recipient address. Error 553 is specifically about the sender's identity; it indicates the "From" address or sender authentication is not allowed, often due to missing SMTP authentication or a relay restriction. Microsoft groups 550 and 553 together in its documentation because both relate to relay-prohibited scenarios, but the fix for 553 is almost always enabling SMTP authentication on the sending side.
Related bounce codes
Sources & further reading
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026
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