Email delivery issues are a pain to deal with. From having bad DNS records to email server problems and reaching your email memory quota, these email delivery problems arise for many reasons.

And one of the most critical series of email error messages is the 5xx one, especially from 550 to 559 error codes. But what does the 550 email error mean, and how do you fix this problem? Let’s talk about that now.

What is 550 Spam Error Message?

550 High Probability of Spam

The SMTP 550 High Probability of Spam is an error message which occurs when you send an email to another mail server, but the latter rejects it, classifying the email as spam.

Although it’s frustrating that the recipient server can automatically classify your message as spam, it is a safety precaution to keep users safe from possible hazards that may pass through the spam filter.

Spam Filter Blocked the Message

When you receive the “550 High Probability of Spam” error, it could also mean that a spam filter has blocked your outgoing message. If that’s the case, your email domain is likely in Realtime Blackhole Lists (RBL).

RBL are directories containing IP addresses, email servers, or domain names that are known to produce, host, help, or forward spam. Use websites like MxToolbox, to find out if your email is listed. If it is, the first thing to do is have it delisted.

Mail Servers Get Triggered

Another reason for the “550 High Probability of Spam” error on a healthy or new domain is that it may have triggered some mail servers.

This error message appears because a part of your email may contain words that trigger the spam filters or the recipient mail server classifies your message as spam. Spam filters usually check for red flags, such as:

  • Emails without an unsubscribe link
  • Messages in ALL CAPS
  • The use of specific words
  • Links to sketchy websites
  • Broken HTML code

DNS or Reputation Issue In Your Sender Domain

Suppose you have warmed up the email address before sending out outreach messages but still received the same error message (550 High Probability of Spam) from the remote server. In that case, a DNS or reputation issue in your sender domain could have caused this.

There are a few things the receiving mail server does when sending an email from one server to another. These receiving mail servers will check the following:

  • The content of your email
  • Your mail sender’s reputation or that of your domain
  • The header part of your email

The receiving mail server checks the header portion of the email to ensure they correctly signed the message. The same server confirms that the email comes from the specified mail server (and not a spoofed email).

Top 8 Solutions to Email Error 550: High Probability of Spam

Let’s review some solutions to prevent this email error and some troubleshooting methods to make it less frequent.

Solution # 1. Set Up SPF, DKIM, and DMARC

If you receive high bounce backs because of SMTP “error 550: high probability of spam,” immediately check if your SPF, DKIM, and DMARC are correctly set up in your domain. These are a set of email authentication protocols that prove you’re authorized to send email from your domain.

Solution # 2. Enable SMTP authentication

Sending an email wouldn’t be possible without SMTP. It’s a protocol used by mail servers to relay outgoing mail from the sender to the recipient. By protocol, we mean a set of rules that allow different email accounts and programs to exchange information.

Most email applications have already automatically configured the SMTP (Simple Mail Transfer Protocol). But if the application did not configure it, that’s when an email error happens. Luckily, it’s easy to configure SMTP manually.

How do you enable SMTP authentication? Open your mail client, check the SMTP configuration panel, and flag the “Authentication Required” option. Then choose the authentication method, set a username, and change your server port to the recommended 587.

Here’s how the process looks in a third-party email application like MS Outlook:

Step 1. Go to Account Settings

Step 2. Choose Server Settings, then Outgoing Mail. The email provider’s SMTP port and the SMTP server will be displayed.

Step 3. Check “My outgoing (SMTP) server requires authentication.”

Step 4. Select the authentication option you prefer.

Step 5. Save your changes.

Solution # 3. Block Direct SMTP Connections

Another way to fix the email error 550 for your inbox is to block spam scripts trying to connect with the remote SMTP servers. By blocking direct SMTP connections, your emails will be sent through mail servers. And doing so makes it easier to monitor the email volume per user and take steps if something suspicious happens.

Solution # 4. Limit The Number of Emails You Send Per Day/Hour

Email and internet service providers pay attention to the number of emails you send daily from a domain. If they find that you send too many emails, it can cause other providers and spam filters to identify your mail activity as spam.

Most ESPs limit the number of emails you can send daily but avoid coming close to such a limit. If you do, it can harm your domain reputation with your own ESP and outside providers.

Check out these tips on how to send mass emails without spam.

Solution # 5. Stop Spamming

You could be a legitimate sender, but your email may still be spam. Spam filters don’t know your intentions. They only consider how you follow the email marketing rules. For instance, it found that you used words that trigger spam filters.

So, if your email marketing practices mimic that of a spammer, you will be tagged as a spammer. As a result, your emails will land in the spam folder instead of the inbox. Your solution, therefore, is to stop spamming or ensure your email contents don’t read as spam.

Ensuring your content doesn’t read as spam is also good as fewer spam messages reach your inbox. We strongly recommend this email deliverability guide for marketers to avoid landing in the spam folder.

Solution # 6. Work on DNS Management

Domain Name System (DNS) is a protocol for exchanging information on the internet. It works like the satellite navigation system in a car. You get in the vehicle, turn on the satellite navigation, and input a destination (name or postcode). The system then converts the destination into accurate GPS coordinates (example: 41°24’12.2″N 2°10’26.5″E.)

In the same way, if you want to visit a site, you turn on your device, open a browser, and type the domain name (example: The Domain Name System translates the name you just typed into an IP address, which identifies the resources linked to the internet.

In email marketing, the mail server uses DNS to complement the email address to its destination and, eventually, deliver the email. DNS is also used to publish Sender Policy Framework (SPF) records to enable an email marketing tool to send newsletters on behalf of its client.

You may lack the proper security settings if you receive the email error 550 messages because of incorrect routing or a high probability of spam. The solution, therefore, is to make the necessary changes to the DNS management setting.

Solution # 7. Raise Your Sending Reputation

If you confirmed that you correctly implemented all the security protocols and checked that your domain doesn’t exist on any blacklist, the culprit for the 550 error message might be having a poor sending reputation.

The most effective way to solve this problem is to raise your sending reputation by pausing outside sender activity using the same domain and doing an inbox warmup. Immediately change the problematic inboxes with another sender domain if you are in the middle of an outreach campaign.

You can use a reliable email deliverability tool to boost your sender reputation, fix delivery issues, and ensure you’re less likely to experience email error 550.

Other platforms allow you to monitor outgoing and incoming mail traffic volume and investigate a sudden drop or any spike in the mail. Or you can opt for tools that analyze mail logs and check the recent changes in the mail server.

Solution # 8. Contact Live Support

If you’ve worked with the solutions above and still have issues with your emails landing in the spam folder, you may contact the online hosting service’s live support that operates your email server.

And if the email delivery issue becomes too much for your team to handle, reach out to email deliverability experts and testers at InboxAlly. We have been helping email senders of all sizes and shapes solve delivery issues like this for years. 

And you know what’s the best part? You can try our tool completely free. We’ll give you access to InboxAlly’s entire suite of features. No credit card is needed. Check us out and watch how we get your email back on track.

Common Phrases Indicating Email Error 550

  1. Email Error 550 Denied By Policy – It happens when the email security software prevents attackers from guessing email addresses when it sends to randomly generated addresses. The email software blocked the outbound emails for the user’s protection.
  2. Email Error 550 Administrative Prohibition – It means the following: (1) the SMTP server has a dynamic IP address; (2) the recipient uses a domain listed on an RBL; and (3) the SMTP server uses a domain on RBL.
  3. Email Error 550 5.1.0 Address Rejected – The recipient’s server blocked your email address and didn’t accept emails from you because it stopped your email address.
  4. Email Error 550 5.1.1 – You entered an invalid or nonexistent email address in the destination domain. It could also be a server misconfiguration or an email address typo.
  5. Email Error 550 5.4.1 – A DNS misconfiguration may cause this error, or the recipient’s server doesn’t accept emails from your domain.
  6. Email Error 550 5.7.0 Message Size Violation – The email contains a file that exceeds the size limit.
  7. Email Error 550 5.7.1 Access Denied – It could be due to an incorrect configuration or issue with security policies on the recipient side.
  8. 550 Error Recipient Quarantined – The email was considered spam, or the email address used is blacklisted.
  9. Email error 550 reverse DNS – This error denotes either a mismatch of DNS records in the domain used to deliver the email or the absence of a Reverse DNS record for the IP address of the mail server.
  10. Email Error No Such User Here or Recipient Unknown – This message indicates that the recipient’s mail server could not find the email account because of an email address typo.
  11. Email error “550 Authentication is required for relay” means missing SMTP authentication. To send outgoing mail, you should enable the Simple Mail Transfer Protocol authentication on the email client.
  12. Email Error 550 Relay Not Permitted – The recipient’s server did not allow the server you used when sending the email.
  13. Error 550 #5.1.0 Address Rejected – This error message pops out if the recipient blocks any Exchange Online emails or your email address.
  14. Email error 550 host unknown – This indicates an unknown hostname in the email address.
  15. 550 Blocked Error – The email will be returned to the sender. It can happen because the recipient’s hosting server blocked the email you want to send.
  16. Error 550 in Outlook – This SMTP error code denotes that your email server requires SMTP authentication before sending out emails, and the email your client used has not yet been authenticated with your password and username.
  17. Server error “550 this domain is not hosted here” – The email is rejected because the recipient’s username is inaccessible or not found. Other reasons include incorrect mail exchange records directing to the wrong server and email address typo errors.
  18. Email error 550 SPF – You will receive this error when you send an email from an IP not recorded in the Sender Policy Framework.

Other Possible Causes for 550 Error Messages

Email Address Doesn’t Exist

Most 550 email errors simply mean sending to a non-existing email address. It’ll appear as the “550 No Such User Here” error. This error happens either because there’s an error on the recipient’s server or due to email address typos.

You get the same 550 error message when:

  • The MX records are incorrect (points to a different server);
  • You have incorrect or not readable mail file permissions; and
  • The email did not route correctly

Incorrect Email Routing

The “550 No Such User Here” error caused by incorrect email routing is not easy to understand as one caused by sending to an incorrect email address. It’s best to know where the error occurred first to figure out why it happened. Here’s how.

When an organization or an individual sends an email, it travels from the point of origin (email client) to its destination. Along the way, the email has to pass through many servers to ensure it arrives in the correct place. Below are the basics of how an email message travels:

Email message flow

  1. Mail user agent (MUA) – You need an email client or MUA to send and receive emails. That email client can be a phone or desktop app (Thunderbird or Outlook) or a webmail client. Unfortunately, some email clients hide their settings or don’t automatically enable email authentication. These reasons can cause incorrect email routing.
  2. Mail transfer agent (MTA) – Whenever you send an email, that email is uploaded to your mail server. You may experience TLS/SSL errors or authentication problems in this part of the email message flow. The user logs in using a valid username and password while the MUA logs in securely using TLS protocol.

An email accepted in the mail server will be delivered to the recipient. Technically, the email was delivered to a Mail Submission Agent (MSA). In turn, the MSA delivers the email to a Mail Transfer Agent.

If the email is delivered to an external user (who exists on the server), the MTA checks the recipient domain’s MX records and provides the message to the server. Email delivery can fail in this phase, and the email will be in a queue. An excellent example of this temporary email error is when the recipient’s inbox is full.

  1. Mail Delivery Agent (MDA) – MDA is the server that receives emails from the MTA. The MDA stores email so that the recipients’ MUA retrieves it. The two protocols for retrieving emails are the Post Office Protocol (POP3) and the Internet Message Access Protocol (IMAP).

IP Blacklisting

Like the “550 High Probability of Spam” error message happens when a domain is in RBL, an error “550 Email Blocked” can occur due to IP blacklisting. It happens when the sender’s server IP has been listed as a spam source in Senderbase, SpamHaus, or other email reputation tracking services.

How to Fix Other 550 Email Errors

550 High Probability of Spam

Have Your Email Address Whitelisted as Non-Spam

If you found that a spam filter blocked your message, the solution is to have your email address get whitelisted.

Visit the RBL that listed your email or IP. Find a link there that allows request removal from the list. RBL usually provides information on why they recorded your email and the time of listing. These data will be helpful for you in detecting what caused the blacklisting.

Once you send the request for delisting your email or mail IP, it may take a few hours to get delisted. Keep in mind, however, that your email will be listed again on the same RBL if there is another spam abuse using your email or IP address.

So, the best action is to know the root cause of the blacklisting and remove that cause. Trojans, spyware, and viruses may also be causing a PC in your network to (knowingly or unknowingly) send spam emails.

If you or your client’s company sends mass emails, like newsletters, these emails may have been reported by a recipient who thought the message was spam.

Getting your mail IP delisted and clearing a spam script infection is a good start. But it would be better to implement preventive measures to avoid another attack.

Make Additional Space or Scale Up the Plan

Suppose you received the “550 Sender Verify Failed” error message. In that case, you have two solutions: to make way for additional storage or scale up the plan so you can exceed the mailbox’s limited storage capacity. Deleting the emails you no longer need gives space for new messages.

Check If Your Account Has Been Compromised

There’s a chance you’ll receive the 550 error message even if you didn’t send the original message. It can happen when a hacker or spammer inappropriately uses your email account to send a message.

To know if your account has been compromised, check the Sent Items, especially the recent messages there. Are there unknown or strange messages – those you didn’t send?

If you find that your account has been compromised, scan your devices for malware or reset your password. But since the spammer may have configured other settings on your mailbox, you can notify your email admin to unblock the account before sending another email.

Fix Spam Link Injection

Attackers exploit web vulnerabilities to upload spam bots and scripts. These spam scripts follow commands from a “bot master” and send based on those remote commands.

The solution to the “550 Email Blocked” errors is to clean out spam scripts. The next step is to determine what web application vulnerability was utilized for the attack and patch the vulnerability to avoid further infection.

Change Your Outgoing Mail Port

If the outgoing mail port is 25, there’s a chance that the server will block the email coming from it. This is because spammers usually use port 25. So, try changing your outgoing mail port to 2525, 587, 465, or 26.

Wrap Up

Email marketing isn’t going to be perfect. It is effective, yes, but not perfect.

But if you can identify your email problems, know the solutions to fix them, and are ready to test those solutions, then all you need to do is get started.

By using the definitive guide above, you’ll quickly solve the 550 email error message. In addition, we encourage you to do an A/B test. It will lead to a better email marketing campaign and make you a better marketer.

Fixing email delivery failures shouldn’t be tough. Want to see how we can fix yours? Request a demo today.