SMTP error 550 indicates that the recipient’s mail server has rejected your email. This error is part of the Simple Mail Transfer Protocol (SMTP), a set of standardized rules for sending and receiving emails. Here are some ways to resolve this SMTP error:  

  • Recheck the email address for typos or errors.  
  • Check if the recipient’s mailbox is active.  
  • Do not use elements like excessive links, suspicious attachments, etc. that might trigger spam.  
  • Check your SMTP settings.  

Let us take a closer look at SMTP error 550, from why it occurs to how to fix it. Read ahead to know more. 

What is Simple Mail Transfer Protocol? 

Before diving into the details of SMTP error 550, let us understand simple mail transfer protocol. SMTP is the standard protocol used for sending emails across the internet. When you send an email, SMTP is responsible for delivering it from your email client (like Gmail, Outlook, or any other service) to the recipient’s mail server. 

SMTP uses a series of commands to send messages between servers. However, sometimes things go wrong during this process, and that is when you see errors like SMTP Error 550. 

What is SMTP Error 550? 

SMTP error 550 is a message that appears when the recipient’s email server refuses to accept the message you are trying to send. Simply put, it is an error message you get when the recipient’s server blocks your email and sends it back.

This error typically appears as a bounce-back message like “your email didn’t reach the intended recipient” or “the email account that you tried to reach does not exist.” Here are some other SMTP error 550 messages you might get:

  • 550 Requested action not taken or mailbox unavailable. This indicates that the recipient’s mailbox is either full, inactive, or does not exist.
  • 550 5.7.1 Unable to relay. This error occurs when the server is not configured to relay messages from your email address. It happens if you are trying to send an email through an SMTP server that does not recognize your domain.
  • 550 5.4.5 Daily sending quota exceeded. This SMTP error 550 message indicates you have exceeded the number of emails you can send in a day.
  • 550 5.2.1 Mailbox cannot receive email. This means that the recipient’s mailbox cannot receive emails, due to being complete or having restrictions.
  • 550 5.7.1 Message rejected due to content restrictions. The email was rejected because it contains content that violates the recipient server’s policies.
  • 550 5.7.1 Message rejected as spam by Content Filtering. This SMTP error 550 message is displayed when the recipient’s server flags your email as spam.
  • 550 5.1.10 Recipient not in directory. This error message indicates that the recipient’s email address is not found in the directory of the recipient’s mail server.
  • 550 5.7.1 Message size exceeds fixed maximum message size. This SMTP error 550 message is displayed when the email is too large to be delivered.
  • 550 Message headers fail syntax check. This error occurs when the email headers are incorrectly formatted. 

What are the Reasons for SMTP Error 550 

Several factors can trigger an SMTP error 550, and understanding the root cause is key to resolving it. Here are some of the most common causes:

1. Invalid Recipient Address

Typically, an SMTP error 550 occurs due to a problem with the recipient’s email address. It could be a typo, a missing character, an outdated email address, or if the email address does not exist or is incorrect. In this situation, the server will not be able to deliver the message, and you will see an error code like “550 User account is unavailable” or “550 No such user here.”

2. Bad Domain Reputation

Your email might also be blocked if your sending domain has a bad reputation. For example, if your email server’s IP address is blacklisted, the recipient’s server may refuse to accept your email. Your sending domain might be blocked due to:  

  • Your email system is compromised  
  • Your email is being used to send spam   
  • There’s malware on your computer sending emails

3. Server Downtime

Sometimes, the issue is not at your end at all. The recipient’s email server could be temporarily unavailable due to maintenance or technical issues. In such cases, the error message might say something like “550 Mailbox does not exist” or “550 Verification failed.” If this is the cause, all you can do is wait for the server to be back online or try resending the email later.

4. Email Policy Violation

Some email servers have strict rules about what kind of emails they will accept. If your email violates these rules, like including spammy language, too many attachments, or suspicious links, it might be blocked by the recipient’s server. This could be reflected in an error message like “550 Policy violation” or “550 Message rejected due to content.”

5. Large Email Attachments

Another reason you might encounter a 550 error is if your email is too large. Many email servers have a size limit for attachments, and if your email exceeds this limit, the recipient’s server might reject it. If this is the case, you might see a message saying “550 Mailbox full” or “550 Message size exceeds limit.”

6. Bad DNS Records

If your mail server has improperly configured DNS records, it can result in a 550 error. Domain Name System (DNS) is responsible for converting domain names into IP addresses. If there is a problem with your DNS records, the recipient’s server will not be able to verify your email, and your message might be blocked.

7. ISP Restrictions 

Your Internet Service Provider (ISP) could also be causing the SMTP 550 issue. Some ISPs block certain email servers or ports to prevent spam. If your ISP is blocking your email server, the recipient’s email server might return a 550 error. You may need to contact your ISP to resolve the issue.

8. Compromised SMTP Port

Another technical issue that could cause a 550 error is if your SMTP port is compromised. SMTP typically uses port 25 to send emails, but spammers commonly use this port. If the recipient’s server detects that you are using this port, it might block your email. You can try switching to a different port, such as 2525, 26, or 465, to resolve this issue.

How to Resolve SMTP Error 550

Now that we have discussed the potential causes of SMTP error 550, let us look at how to fix this error. Here are the steps you can take to resolve the issue:

  • Check the Recipient’s Email Address 

Before sending an email, double-check the recipient’s email address to ensure it is accurate. Even a small typo can cause the email to bounce back. If you are unsure, contact the recipient to confirm their email address.

  • Verify Your Email Configuration 

If your SMTP 550 error displays “authentication is required for the relay,” it is likely due to incorrect email configuration. The first step to resolve this error is to check that your SMTP settings (server address, port number, and authentication settings) are correct. Additionally, ensure you are using the correct username and password for authentication.

  • Review Your Email Content 

Sometimes, the problem lies in how your email is structured. Spam filters are triggered by specific elements, such as too many links, suspicious file attachments, or text written entirely in capital letters. To avoid being flagged, ensure your email is professional and straightforward.  includes a clear subject line and an option for the recipient to unsubscribe if it is a marketing email. Ensure it includes a clear subject line, looks legitimate, and improves its chances of being delivered. It includes a clear subject line that looks legitimate and improves its chances of being delivered.

  • Update Your Domain’s SPF Records 

Sender Policy Framework (SPF) records are like a stamp of approval for your email. They tell the recipient’s server that your email is coming from a verified and trusted source. The email may get blocked if your SPF record is not correctly set up. To fix this, you can use SPF record-checking tools, which make it easy to verify if everything is configured properly. Updating your SPF records ensures your emails are recognized as safe.

  • Verify Your Domain or IP for Blacklisting 

If your domain or email server has been flagged for spam, your emails might get blocked automatically. This usually happens when the server detects suspicious activity, like bulk or spammy emails. You can check if you have been blacklisted using online blacklist checkers. If you are on a blacklist, you will need to submit a request to be removed. Ensuring your domain is clean will help avoid future delivery problems.

  • Watch Your Sending Limits   

Email providers often have limits on how many emails you can send in a day. If you send many emails and hit this limit, your messages may bounce back. To avoid this, either reduce the number of emails you send or consider upgrading your email service plan. A higher-tier plan often comes with a larger or unlimited sending quota.

  • Contact the Recipient’s Mail Server Administrator   

If you have tried all the above steps and the issue persists, you may need to contact the recipient’s mail server administrator. They can give you more specific information about why your email is being blocked. In some cases, they may need to add your email address to a whitelist or adjust their server’s spam filters to allow your emails to go through.

Conclusion 

SMTP error 550 can be frustrating, but it is usually fixable once you understand the cause. By following the solutions outlined above, you should be able to troubleshoot and fix most instances of SMTP error 550.  

If you are still having trouble, do not hesitate to reach out to your email service provider for further assistance. They can help you identify the cause and get your emails back on track. 

If you still get SMTP error 550 messages, you can switch to a reliable email hosting service. A professional email hosting provider, like BigRock, can help avoid common errors by offering advanced spam filters, secure servers, and proper email authentication setups. Additionally, when you opt for our email hosting service, you get access to 24/7 customer support, which helps diagnose and resolve email-related problems efficiently. 

Still confused? Refer to our FAQs for more clarity!  

Frequently Asked Questions (FAQs)  

  • Can an error in the recipient’s email address or server trigger an SMTP 550 error?  

Yes, SMTP error 550 can sometimes be related to the recipient’s errors, such as incorrect SMTP settings, sending larger emails, or sending from a domain that is not authenticated or is blacklisted.  

  • What can I do to avoid SMTP 550 errors in future?  

It is difficult to avoid SMTP errors completely. However, you can maintain accurate email lists, adhere to email best practices, and make sure your email server settings are properly configured to reduce the likelihood of encountering this error. 

  • How do I check if my domain has been blacklisted?  

You can use online blacklist-checking tools to see if your domain has been blacklisted.  

Still confused or want to share your thoughts? Leave a comment below!