The SMTP error code 550 signals that the requested action couldn’t be completed because the recipient’s mailbox is un­avail­able. This error belongs to the 5XX status code class, in­di­cat­ing a permanent issue, such as an invalid email address, in­suf­fi­cient access rights or a policy violation. In modern mail en­vi­ron­ments, the 550 error can also occur due to spam suspicion or au­then­ti­ca­tion issues.

What are SMTP status codes used for?

To exchange messages, email servers and clients use the Simple Mail Transport Protocol (SMTP). This protocol, part of the Internet protocol suite and defined in RFC 5321, works alongside more familiar protocols like POP3 and IMAP, which are used ex­clu­sive­ly for re­triev­ing email once it’s been delivered.

During SMTP com­mu­ni­ca­tion, servers send status codes to confirm con­nec­tion es­tab­lish­ment or message transfer, request further input, or indicate a dis­con­nec­tion. Ad­di­tion­al­ly, there are error codes: temporary errors receive codes in the 4XX range, while permanent or critical failures, such as failed de­liv­er­ies, are marked by 5XX status codes. For example, a mail server may return SMTP 550 (sometimes with messages like “Relay not permitted”) when it can’t deliver a message to the intended recipient.

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What causes the SMTP 550 error?

According to RFC 5321, the standard format of the 550 status code is “550 Requested action not taken: mailbox un­avail­able.” This typically means the recipient address is not available. According to the IETF document, some possible causes include:

  • An invalid recipient address (mailbox not found).
  • Lack of per­mis­sions to use the SMTP service.
  • Rejection due to policy re­stric­tions.
  • Mis­con­fig­ured DMARC/SPF/DKIM (e.g. SPF check failed).

This error appears when an email client tries to send a message via SMTP, but the request fails for one of the reasons above. Whether and how the SMTP server com­mu­ni­cates the specific reason to the client depends on the system and software in use — so while the issue is the same, the error message might appear in different forms.

What do SMTP 550 error messages look like?

The message that ac­com­pa­nies an SMTP 550 error can vary depending on the email provider. That’s because the basic 550 code itself doesn’t explain the cause — it only states that the request failed. If you’re trou­bleshoot­ing an SMTP 550 error, it’s important to read the full message carefully. Some common variants include:

  • 550 Requested action not taken: mailbox un­avail­able
  • 550 User unknown
  • 550 Mailbox not found
  • 550 Relay not permitted
  • 550 Recipient address rejected: User unknown in local recipient table
  • 550 Reject due to policy re­stric­tions
  • 550 Invalid address
  • 550 User has too many messages on the server.
  • 550 Permanent failure for one or more re­cip­i­ents
  • 550 SPF check failed
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How to fix a SMTP 550 error

As shown above, the SMTP 550 error has many possible causes, so there’s no one-size-fits-all solution. Whether you can resolve the issue yourself depends on the situation. Below, we cover solutions to four common vari­a­tions of the SMTP 550 error.

How to solve the au­then­ti­ca­tion “550 Relay not permitted” problem

The message “550 Relay not permitted” indicates a failed con­nec­tion between your email client and the mail server (some servers use SMTP 553 for this). Two common causes include either because SMTP au­then­ti­ca­tion is not enabled in the client, or the stored cre­den­tials are incorrect.

In both cases, you’ll need to check and update the outgoing server settings in your email client. If you’re using Outlook, you can do this as follows:

  1. Go to the Control Panel, then select Mail (Microsoft Outlook).
Image: Windows Control Panel showing the mail option
Mail interface in Windows Control Panel.
  1. Click on E-Mail Accounts, choose your mailbox, then click Change to access the POP and IMAP account settings. However, you don’t need to make changes here. Click on the More Settings button to open the Internet E-Mail Settings menu.
Image: POP and IMAP Account Settings in Outlook
In the POP and IMAP account settings, you'll find all important data about your Outlook account and the mail server being used.
  1. In the Outgoing Server tab, check the box labeled My outgoing server (SMTP) requires au­then­ti­ca­tion. Also select Use same settings as my incoming mail server so Outlook can operate with the usual login details. If au­then­ti­ca­tion was already enabled, but uses different cre­den­tials, switching to this default login option may resolve the issue.
Image: Outgoing Server Options in Outlook
If you have changed the settings for outgoing server au­then­ti­ca­tion in your Outlook client, you must confirm these changes by clicking 'OK'.

How to fix the “550 requested action not taken mailbox un­avail­able” message

This variation of the SMTP 550 error usually indicates that the recipient address cannot be reached. In such cases, the first thing to check is the address itself – often, a simple typo or mistake while creating the email is to blame. However, the issue can also be on the recipient’s end. It might occur if the recipient’s address is tem­porar­i­ly un­reach­able due to a technical issue or if the mailbox has been de­ac­ti­vat­ed or deleted.

If possible, contact the recipient by another method to confirm the correct address or ask them to re­ac­ti­vate their mailbox. If that’s not an option, try resending the message later — the recipient may become reachable again.

How to fix the “550 reject due to policy re­stric­tions” message

This error means that some aspect of the email violates your provider’s sending policies. The most frequent causes are:

  1. Your email address has been flagged as a spam source, leading to blocked delivery.

  2. The sending server address isn’t a Fully-Qualified Domain Name (Fully-Qualified Domain Name).

If the issue is the domain name, ensure the server uses a complete and valid FQDN. If your address is suspected of spam, check it using tools like the Spamhaus Blocklist Removal Center.

Image: Screenshot of the Spamhaus Domain Check
At spamhaus.org, you can check not only domains but also IP addresses.

Enter the domain and click Lookup. The tool checks against the Spamhaus database of known spam sources and tells you whether you’re listed or not.

Image: Screenshot of the Spamhaus Domain Check Result
Even if your domain isn't listed, spam-related issues may still be the cause of the SMTP 550 error.

If you are listed, contact your email provider im­me­di­ate­ly to resolve the issue together.

How to fix the “550 SPF check failed” message

This error means that the recipient’s mail server checked your message against the SPF record of your domain and found that the sending server wasn’t au­tho­rized to send emails on behalf of the domain. SPF is a DNS-based mechanism that allows domain owners to specify which servers are au­tho­rized to send emails on their behalf. Since May 5, 2025, major providers like Microsoft have enforced SPF, DKIM, and DMARC checks more strictly. If an email is sent from a server not listed in the sender domain’s SPF record, the receiving server may reject it with the error code 550 5.7.15.

To fix the SMTP 550 error:

  1. Check your SPF record using MXToolbox SPF Lookup to verify if the sending server is listed in your domain’s SPF zone.
Image: Screenshot of the MX Lookup Tool by MXToolbox
Enter your domain to check your SPF entries.
  1. Update your SPF record if your sending server is not listed. Here is an example SPF entry for IONOS:
v=spf1 include:_spf.perfora.net include:_spf.kundenserver.de ~all
txt

Note that DNS changes may take 24 to 48 hours to take effect globally.

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