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How to Fix the error "550 High Probability of Spam": The 4 Solutions.

Damien Brenelière
Co-Founder @ MailReach & Email Deliverability Expert

Finding bounced emails in your inbox that say 550 high probability of spam is not reassuring. Don't worry, we'll explain you why it's happening and the best ways to fix that ASAP.

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Why am I having the error 550: High Probability of Spam? The causes.

The "550 High Probability of Spam" email error is usually returned by the recipient's email server or inbox provider when it determines that the message you are sending is likely to be spam. This can happen for several reasons. Let's jump in.

Your email content is flagged as spam by the recipient's spam filters.

The email error 550 high probability of spam can be returned by mail servers because of your content.

Content related spam is pretty common as spam filters / inbox providers check your message to decide to put it in the trash folder or not.

The top causes of having your message content trigger spam filters are :

  • Attachments to people you never contacted before
  • Including links / URLs (like your website, tracking domain, external websites)
  • Spam words / sensitive topics :
  • Money earning / growth / salary / saving / investment / stocks / sales / money games
  • Loans / mortgage / credit / debts / real estate / insurance
  • Promotions / discounts
  • Security / cybersecurity
  • Health / diet / weight loss / sex
  • Aggressive punctuation
  • CAPITAL LETTERS
  • Many others
  • Including a too high proportion of image in your message

You are sending a high volume of emails in a short period of time, which can be a sign of spamming.

Sending too many emails per day is already a bad practice that damages your sender reputation over time but a too high volume in a very short period of time will trigger spam filters and errors like this even more.

The fewer emails sent per day, the better for your deliverability and reputation.

For a cold email activity, we recommend a maximum of 150 per day over several hours and avoid one shot blasts.

More information below.

Your sender reputation (mailbox, domain and IP) is too low.

Nowadays, your sender reputation is mostly based on the engagement rate of your email activity.

In other words, if you get a lot of positive interactions on your email activity (replies, openings, having your emails marked as important and removed from spam) compared to the volume you're sending, then your sender reputation will be good.

On the opposite, if your engagement rate is low (very few openings, replies, having your emails marked as spam, having too many bounced messages), then your reputation will be low.

And after all, that makes sense. It's for now, the best way for mail servers and inbox providers to check if you're a legit, interesting sender who sends relevant emails to your recipients.

If globally, your recipients don't react positively to your emails, it says to mail servers that you're a spammer and deserve to land in trash, or worst, land nowhere. In the latter, that's a shadow ban.

Here are the 4 (best) solutions to solve this error and minimize the risk to face it again.

The error 550 high probability of spam returned by mail servers can be fixed and your risk of facing it again minimized by following the instructions below.

1. Run a deliverability test using a proper spam testing tool (we have what you need and it's free).

First things first. You can't improve what you don't measure.

Your first step should be to run a deliverability test to measure your deliverability and get a first diagnostic of your message content and setup.

Don't use any deliverability test out there as most of them give biased results.

The only reliable way to measure your deliverability is to send an email to a significant list of addresses that represent your target recipients, check where you land and get a helpful report on what you can improve.

To get a reliable result, use our free email spam test. You can run up to 3 free tests in 24 hours.

The report will give you a score and tell you exactly where your email landed on Google inboxes (Gmail and Google Workspace), Microsoft (Outlook and Office 365) and Yahoo.

You'll also be able to see if there are elements that can be improved : words / sentences, links, tracking, DNS records and so on.

Once you get your first report, make changes to your email and/or setup to see if you manage to get a better inbox placement.

2. Limit your daily volume of emails to avoid the spam folder.

To fix the error 550 high probability of spam returned by email servers, you need to check your volume.

For a cold email activity or simply normal activity.

If you're sending prospecting / cold emails, your maximum per address per day should be around 150.

This number is based on the data collected by MailReach over several years. Again, the fewer the better and the safer for your reputation and deliverability.

Also, the total volume per domain counts. The fewer per domain the better.

Again, we actively recommend to read our guide : The Best Cold Email Deliverability Sending Strategy to Avoid Landing in Spam.

For an opt-in, marketing or transactional email activity.

For that use case, it's obviously more difficult to minimize your volume per address and domain.

But if your recipients have opted in, you should be able to land in inbox, as long as you have enough engagement (positive interactions) on your activity.

If your recipients didn't opt in, that's another story and that will put you in the trash folder since you'll get a too high volume with a too low engagement rate leading to a reputation and thus deliverability drop.

3. Make sure your SPF, DKIM and DMARC are properly set up to land more often in inbox.

SPF, DKIM are mandatory to land in inbox and avoid errors like 550 high probability of spam returned by email servers.

DMARC is not yet mandatory but we still recommend to set that up as it may become more important in the future.

Our deliverability test will tell you if your SPF, DKIM and DMARC have been properly set up.

4. Use an email warming solution to raise and maintain your sender reputation.

As said earlier, to make sure your emails land in the main inbox, you need to have a high sender reputation.

To have a high reputation, you need to show the mail servers and big inbox providers like Google and Microsoft that you receive enough positive interactions to your emails and have a high engagement rate.

That's exactly what does an email warm up service : generating a lot of engagement on your email activity by sending emails to a network of addresses (MailReach's network, composed of human inboxes) and getting a lot of positive interactions : positive replies, openings, marked as important and removed from spam.

MailReach helps you reach the inbox and stay there by constantly generating engagement to your sending addresses and domains, and help you optimize your email content to pass the spam filters with a powerful spam checker tool.

MailReach is compatible with any inbox / SMTP

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