You craft the perfect email, hit send, and wait for responses—but nothing happens. No replies, no engagement, just radio silence.
Then, you check your email logs and see this dreaded message: “550 High Probability of Spam.”
Frustrating, right?
Your emails are being blocked before they even reach the recipient’s inbox. And worse—this error can damage your sender reputation, making future emails even more likely to get flagged as spam.
But don’t worry—you’re not alone, and this problem is 100% fixable.
In this guide, you’ll learn:
- Why the “550 High Probability of Spam” error happens
- How spam filters decide which emails to block
- Proven fixes to restore your email deliverability
What is the “550 high probability of spam” error?
The “550 High Probability of Spam” error occurs when a recipient’s mail server flags an email as potential spam and refuses to deliver it. This happens when the email fails security checks, such as SPF, DKIM, or DMARC authentication, or when the sending domain has a poor reputation.
Mail servers also analyze email content, formatting, and sending behavior, comparing them against known spam patterns. If an email contains spam-triggering keywords, excessive links, or comes from an untrusted source, it may be blocked to protect users from phishing and unsolicited emails.
Common Reasons Why Emails Get Flagged as Spam
Some of the most common triggers that increase the chances of an email being flagged include:
- Emails without an unsubscribe link – Many spam filters prioritize CAN-SPAM compliance, so missing this can be a red flag.
- Messages in ALL CAPS or excessive punctuation (e.g., “FREE!!!”) – Spam filters are designed to catch excessive capitalization, symbols, and overuse of promotional language.
- Overuse of spam trigger words – Certain words like “limited-time offer,” “free trial,” and “urgent” have been associated with higher spam filtering rates in deliverability tests.
- Poor sending practices – This article by Spamhaus explains how poor sending practices, such as not managing bounces and sending emails without proper consent, can lead to spam listings
- Links to blacklisted domains – If your email contains links to domains with a bad reputation, spam filters may block it automatically.
- Poor HTML formatting – Emails with broken or excessive HTML, missing headers, or unnecessary embedded scripts can look suspicious.
A proper email authentication (SPF, DKIM, DMARC) is crucial for a good sender reputation, and by avoiding spam-like language, you can reduce the chances of your emails being blocked and improve deliverability.
How to Fix the “550 High Probability of Spam” Error
Now that we understand the causes let’s review proven ways to fix this error and improve email deliverability.
1. Configure SPF, DKIM, and DMARC
If you’re getting the “550 High Probability of Spam” error, check that your domain has the correct SPF, DKIM, and DMARC records. These are a set of email authentication protocols that prove you’re authorized to send email from your domain. Microsoft states that failing to configure SPF, DKIM, and DMARC properly can result in email rejection or filtering into spam folders, especially for Office 365 users.
To set up SPF, add the following TXT record in your domain’s DNS settings:
- Google Workspace: v=spf1 include:_spf.google.com ~all
- Microsoft Outlook: v=spf1 include:spf.protection.outlook.com -all
- Zoho Mail: v=spf1 include:zoho.com ~all
For DKIM, generate a DKIM key from your email provider’s admin panel and add the TXT record to your DNS. Google Workspace and Microsoft Office 365 have built-in DKIM management tools.
For DMARC, add this TXT record:
v=DMARC1; p=quarantine; rua=mailto:your-email@example.com
Use tools like MxToolbox or Google Admin Toolbox to verify your records. If emails are still flagged as spam, check your domain’s reputation and make sure the records were properly applied.
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:
- Go to Account Settings
- Choose Server Settings, then Outgoing Mail. The email provider’s SMTP port and the SMTP server will be displayed.
- Check “My outgoing (SMTP) server requires authentication.”
- Select the authentication option you prefer.
- Save your changes.
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. Doing so makes it easier to monitor the email volume per user and take steps if something suspicious happens.
4. Control Your Email Sending Limits
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.
5. Avoid Spammy Email Practices
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.
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.
Ensuring your content doesn’t look like spam is also good as fewer spam messages would reach your inbox. We strongly recommend reading this email deliverability guide for more info.
6. Optimize DNS Settings
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.)
Likewise, if you want to visit a site, you turn on your device, open a browser, and type the domain name (for example: inboxally.com). 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.
7. Improve Your Sender 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.
A case study from InboxAlly shows how Agexa, a digital marketing agency, recovered from a 5% open rate caused by a poor sender reputation. By implementing InboxAlly’s warmup strategy and gradually rebuilding their reputation, they increased inbox placement by 77% and improved open rates to 41% within weeks.
8. Get Professional Support from InboxAlly
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 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.
Troubleshooting for Different 550 Error Variants
If you’re encountering a 550 email error, it may not always be due to blacklisting or missing authentication records. There are several other reasons why your emails might be rejected, and understanding these factors can help in troubleshooting and resolving the issue.
1. Invalid Recipient Address
Most 550 email errors occur when sending to a non-existent email address, appearing as the “550 No Such User Here” error. This can happen due to:
- A typo in the recipient’s email address.
- The recipient’s server rejected the message due to an internal issue.
- Incorrect or unreadable mail file permissions.
- Misconfigured MX records pointing to the wrong mail server.
2. Email Routing Issues
The “550 No Such User Here” error caused by incorrect email routing is more complex than an issue with an invalid email address. To troubleshoot, it’s essential to understand where the error occurred during the email’s journey.
How Email Routing Works:
- Mail User Agent (MUA): This is the email client used to send and receive emails (e.g., Outlook, Thunderbird, or a webmail client). Some clients fail to enable authentication by default, which can cause routing issues.
- Mail Transfer Agent (MTA): When an email is sent, it gets uploaded to the sender’s mail server. TLS/SSL authentication errors or incorrect SMTP login credentials can block delivery at this stage.
- Mail Delivery Agent (MDA): The MDA is responsible for storing the email until it is retrieved by the recipient’s client (via POP3 or IMAP). If there is an issue at this stage, the email might remain in a queue or be rejected.
3. Message Processing Errors
Some email servers reject messages due to improper formatting, large attachments, or encoding issues. Common problems include:
- Oversized attachments exceeding the recipient’s allowed limit.
- Improperly formatted email headers that violate RFC standards.
- Issues with HTML emails, such as missing tags or incorrect encoding.
4. IP Blacklisting
Like the “550 High Probability of Spam” error message that occurs when a domain is listed in an RBL (Realtime Blackhole List), the “550 Email Blocked” error can occur due to IP blacklisting. This happens when an email sender’s IP is flagged as a spam source by services such as:
- Spamhaus
- SenderBase (Cisco Talos)
- Barracuda Reputation Center
If your IP has been blacklisted, improving your sender reputation is the first thing you need to do to avoid future blocks. InboxAlly can help restore inbox placement by gradually rebuilding trust with email providers—go here to learn more.
5. Whitelist Your Email Address
If a spam filter blocks your message, the best solution is to have your email address whitelisted.
How to Request Removal:
- Check the RBL (Realtime Blackhole List) that flagged your email or IP.
- Find the removal request form on their website (Spamhaus, Barracuda, or Cisco Talos).
- Review the reason for the listing and resolve the underlying issue before requesting delisting.
- Once you submit the request, delisting may take a few hours.
Important: Your email will be blacklisted again if spam activity continues. Ensure no unauthorized emails are being sent from your domain.
6. Free Up Mailbox Space or Upgrade Storage
If you receive the “550 Sender Verify Failed” error message, it could mean the recipient’s mailbox has exceeded its storage capacity.
Solutions:
- Delete old emails to free up space.
- Upgrade to a larger mailbox plan if necessary.
7. Check for a Compromised Account
If you receive a 550 error message for emails you didn’t send, your account may have been hacked.
How to Check:
- Look at your Sent Items for emails you didn’t send.
- Scan your device for malware or spyware.
- Reset your email password and enable two-factor authentication.
If the hacker made unauthorized changes to your email settings, notify your email admin to investigate further.
8. Remove Spam Link Injections
Cybercriminals exploit website vulnerabilities to insert spam scripts into emails. These scripts can cause mass spamming, leading to IP blacklisting.
Fix:
- Remove any malicious scripts from your website.
- Patch security vulnerabilities to prevent further attacks.
9. Change Your SMTP Port
If your outgoing mail port is 25, there is a chance that mail servers will block your emails, as spammers frequently use port 25.
Alternative SMTP ports to use:
- 2525
- 587
- 465 (SSL-enabled mail servers)
Switching to an alternative port may resolve 550 errors related to SMTP blocks.
To fix 550 email errors, it’s important to identify the cause. Addressing these errors with proper authentication, reputation monitoring, and server configurations ensures better deliverability and reduces the chances of future rejections.
If you’d prefer to leave your sender reputation in expert hands, InboxAlly can help optimize inbox placement and keep your emails out of spam. See how it works here.
Conclusion
Spam filters and deliverability issues can be frustrating, keeping your emails from reaching the right audience. While proper authentication and best practices help, staying ahead of ever-changing filters is a challenge.
That’s where InboxAlly comes in. Our deliverability tool helps you land in inboxes, rebuild sender reputation, and keep your emails out of spam—without the guesswork.
Don’t let undelivered emails cost you opportunities. Try InboxAlly for free today and start reaching your audience where it matters most.