Are Blacklists Killing Your Emails? A Deep Dive into How They Influence Email Providers

Let’s take a closer look at each provider.

Google Workspace (Business Gmail)

Internal machine learning takes priority for spam detection. Deliverability is based on behavior, engagement, and reputation—not external listings.

How spam filtering works

Spam filtering in Google Workspace is powered by advanced machine learning models trained on billions of messages and users’ behaviors. These models analyze signals like sender domain/IP behavior, SPF/DKIM alignment, bounce patterns, user engagement, and spam complaints.

Blacklist usage

Google Workspace does not use DNSBLs like Spamhaus or Barracuda to filter emails in most cases. Since Google joined the email game a little late, they chose to rely on their big data approach for assessing reputation and detection of spam. However, there are some observations:

• There are theories suggesting that Google factors in Spamhaus data in their algorithms. Senders who get listed on Spamhaus also often notice Gmail deliverability issues around the same period. Most likely, the bad sending practices that triggered Spamhaus also triggered Gmail’s spam filters. 
• The only known integration is the Spamhaus PBL, which is used to block mail from dynamic IPs or unauthorized senders. This is more of a policy enforcement than a spam detection method.

Bottom line: blacklists are important, but not the deciding factor

If you’re listed on Spamhaus PBL, your emails may be outright rejected with a clear SMTP error. However, being on any other blacklist won’t automatically hurt your Gmail deliverability. Instead, poor sender reputation or low engagement will.

Gmail (consumer Gmail)

Gmail’s free service uses the same infrastructure as Google Workspace. The spam filtering is entirely automated and behavior-based.

How spam filtering works

Gmail’s adaptive spam filter uses internal data and user signals across its 1.5+ billion users. It looks for indicators like reply rates, bounce rates, complaint frequency, and consistency in authentication records.

Q: What are spam filters?

A: Spam filters are automated systems used by email providers to detect and block unwanted or potentially harmful emails. They analyze factors to decide whether a message goes to the inbox, spam folder, or gets rejected.

🔖 Related Reading: Spam filters: everything you need to know

Blacklist usage

Like Google Workspace, Gmail doesn’t rely on external blacklists. The only exception is the Spamhaus PBL, which Gmail uses to reject direct sends from unauthorized or dynamic IPs. 

Gmail had previously appeared on smaller blacklists due to spammers. However, Gmail doesn’t block mail from its own IPs, which shows that Gmail doesn’t subscribe to third-party blacklists. 

Bottom line: being blacklisted is not the only factor that affects deliverability

Your email can still land in spam if your sender behavior is weak, even with no blacklist listings. Conversely, being listed on won’t stop Gmail delivery unless it correlates with poor practices. Blacklist removal is still good hygiene, but it’s not a silver bullet for Gmail deliverability.

Microsoft 365 (Exchange Online Protection)

Microsoft 365 takes a hybrid approach, combining internal scoring systems with trusted third-party data—most notably Spamhaus ZEN.

How spam filtering works

Microsoft uses Exchange Online Protection (EOP), backed by machine learning, user engagement tracking, and SNDS data. It detects spam by analyzing IP reputation, authentication results, spam trap hits, and complaint data.

Q: What are spam traps? 

A: Spam traps are email addresses specifically created or repurposed to catch spammers. They don’t belong to real users and are used by blacklist operators to identify senders using poor list hygiene or sending unsolicited emails.

🔖 Related Reading: What is a Spam Trap & How to Avoid It

Blacklist usage

EOP does incorporate DNSBLs. If your sending IP is listed, your emails will often be rejected outright during the SMTP handshake, with bounce messages sometimes referencing Spamhaus or linking to delisting resources. 

Specifically, here’s how Microsoft 365 uses blacklists:

• Microsoft 365 actively uses Spamhaus ZEN, including SBL, XBL, and PBL, to block spam at the SMTP level. Emails from listed IPs are often rejected outright. 
• Microsoft does not officially use other third-party DNSBLs like UCEPROTECT. These aggressive blacklists are generally avoided to prevent false positives.
• In addition to Spamhaus, Microsoft maintains its own internal blacklists based on spam complaints, engagement data, and SNDS feedback. 

Bottom line: keep an eye on Spamhaus

If your IP is listed on Spamhaus, your emails to Microsoft 365 users will be rejected or placed in Junk. But take note that even if your IP is not listed, poor engagement or user complaints can still land you on Microsoft’s internal blocklist.

Outlook.com (Hotmail/Live)

Outlook.com (Microsoft’s consumer mail) mirrors Microsoft 365’s filtering policies with added rate-limiting and behavioral scoring.

How spam filtering works

Outlook uses the same backend as Office 365 with additional filters tailored for free users. Aside from built-in rate limiting, it applies machine learning and user feedback (spam reports, engagement) and throttles unknown or suspicious senders. Outlook also applies aggressive rate limiting and internal scoring, especially for new senders or domains with low engagement. You may encounter 4.7.0 deferrals if your sender behavior is flagged—even without a DNSBL listing.

Blacklist usage

As with Microsoft 365, Spamhaus ZEN is heavily used, and listed IPs can be blocked with SMTP 550 5.7.1 errors. There is no official statement whether or not Outlook uses UCEPROTECT even though there were previous rumors. Thus, the consensus: Spamhaus is the primary third-party blacklist of consequence for Outlook.

Bottom line: keep an eye on Spamhaus

Spamhaus listings can get Outlook to block your emails entirely. You may also face temporary throttling (4xx responses) even if you’re not blacklisted, especially with new or cold IPs.

Yahoo Mail & AOL

Yahoo and AOL (operated under the same infrastructure) are more transparent about blacklist usage than most providers—and rely heavily on Spamhaus.

How spam filtering works

Yahoo uses its inhouse SpamGuard filtering engine, which combines machine learning with IP/domain reputation, user feedback, and historical engagement data (whether Yahoo users mark emails as spam or not spam). Since 2017, Yahoo and AOL became part of the same umbrella with AOL’s filtering mechanisms believed to be merged with Yahoo’s systems. 

Blacklist usage

Yahoo actively uses Spamhaus SBL, XBL, and PBL, and this has been publicly announced since 2008. They return explicit bounce messages when a sender is blacklisted—making it easier to identify and resolve. That being said, Spamhaus plays a crucial role in Yahoo’s filtering mechanisms. If an IP is on the list, Yahoo will reject emails from it and send a corresponding SMTP error message.

Other than Spamhaus, there is no evidence that Yahoo uses other third-party blacklists. Beyond blacklists, Yahoo is highly sensitive to low engagement, high volume, and spam complaints. So if a sender’s presence on other blacklists coincides with poor sending practices, Yahoo’s own filtering system might still send the emails to spam.

Bottom line: Spamhaus is a dealbreaker

Being listed on Spamhaus means hard bounces and blocked delivery to Yahoo/AOL addresses. Additionally, even clean senders may get deferred or spam-foldered if engagement is low or complaints rise.

The real deal with Spamhaus vs. other Blacklists

While dozens of blocklists exist, only a few carry significant weight with major mailbox providers—and one stands clearly above the rest: Spamhaus.

Why Spamhaus dominates

Spamhaus is the most widely trusted and integrated third-party blacklist across the email ecosystem. Its influence is so broad that getting listed can instantly derail your entire deliverability strategy. For example:

• Microsoft 365, Outlook, and Hotmail will reject or throttle emails from Spamhaus-listed IPs at the connection level.
• Yahoo and AOL will keep blocking emails as long as the sender is on the list.
• Other providers beyond the big players also reference Spamhaus

What about SORBS, UCEPROTECT, and Barracuda?

While Spamhaus dominates, other DNSBLs have a more limited influence on deliverability:

• UCEPROTECT: This blacklist has drawn criticism for overly aggressive listings and pay-to-delist practices. It includes multiple levels (L1, L2, L3), with L2 and L3 often impacting entire networks unfairly. Most major providers—especially Gmail and Microsoft—ignore UCEPROTECT entirely, or treat it as a low-priority signal.
• Barracuda BRBL: Used mostly by organizations running Barracuda security appliances, not by Gmail, Outlook, or Yahoo directly. If your email is being blocked due to Barracuda, it’s usually happening at a corporate gateway, not a consumer mailbox provider.

The practical guide to dealing with blacklists

Blacklists are a reality of email sending—but getting listed isn’t the end of the road. With the right tools and timely action, you can detect problems early and resolve them before they damage your reputation or deliverability.

How to detect if you’ve been blacklisted

Many senders don’t realize they’re blacklisted until they notice declining engagement or bounce errors. Catching issues early can save your campaigns. Here are some concrete actionable steps you can take to know if you’re blacklisted.

• Be proactive when it comes to blacklist monitoring. Regularly check your sending IPs and domains against known DNS-based blocklists. Warmy’s free email deliverability test actually shows which blacklists you’re listed on. This is especially recommended for senders conducting email warmup.

• Read and analyze bounce messages and SMTP error codes. Do not take these for granted. SMTP error messages often reveal the issues affecting your deliverability—and being blacklisted may be one of them. Look for mentions of Spamhaus in the error messages or codes like 5.7.1.

• Check in with your provider’s Postmaster tools. Gmail Postmaster and Microsoft SNDS will not directly tell you you’re blacklisted. However, they will show if your reputation drops which you should take as a sign to check for any issues. Yahoo’s postmaster tools provide support for delisting requests if you suspect you’re blacklisted.

• Monitor your IP and domain reputation. Some blacklists target domains aside from IPs, and being included can impact your inbox placement negatively. 

How to go about the delisting process

Finding out you’ve been blacklisted can be frustrating. However, do know that there is a solution. Getting delisted from a blacklist starts with understanding why you were listed—and following the appropriate removal steps. 

Here’s how to handle the most common DNSBLs: