In 2025, 40% of emails never reached a visible inbox, according to tests conducted on millions of sends by the Unspam platform (January 2026). Among the causes: blocklists, these databases that indicate suspicious IPs and domains to email servers. The issue is not ignoring them, but rather monitoring all of them without distinguishing between the important ones and those that are dead or ignored by major providers.
SORBS, one of the most cited lists in deliverability guides, permanently closed in June 2024. Dozens of monitoring tools still check it. Much noise, little signal.
What an email blocklist really is
An email blocklist, also called a DNSBL (DNS-Based Blackhole List) or IP black list, is a real-time queried database used by receiving servers. When an email arrives, the recipient’s server sends a DNS query: “Is this IP or domain listed as a spam source?” If yes, the message is rejected or filtered.
There are over 200 active blocklists. They don’t all carry the same weight. Some cover billions of mailboxes, while others are used by only a few thousand private servers, if at all.
What most guides forget: the difference between the lists that have a direct impact on Gmail, Outlook, Yahoo, and Orange and those with marginal reach. Being listed on Spamhaus ZEN is not the same as being on UCEPROTECT L3.
The blocklists to prioritize monitoring
Spamhaus operates the most widely used lists in the world. Microsoft 365, Outlook, Yahoo Mail, and most corporate filters query its lists directly.
- SBL (Spamhaus Block List): Known spammer IPs, manually maintained by OSINT analysts. Between 30,000 and 40,000 active entries.
- XBL (Exploits Block List): Compromised IPs, infected machines, botnets, open proxies. Automatically updated.
- PBL (Policy Block List): IP ranges that shouldn’t be sending emails directly (residential IPs, DHCP). Used to block non-server sends.
- ZEN: combination of SBL + XBL + PBL. This composite list is the one most servers practice query.
Being listed on ZEN can block emails at Outlook and Yahoo at the SMTP level, even before content analysis. Deliverability rates can drop from 90% in a matter of hours.
The Barracuda Reputation Block List (BRBL) is worth monitoring but within a specific scope: organizations using Barracuda Networks appliances. It is not queried by Gmail or Microsoft 365 natively. It is relevant for B2B sends to companies with specific filtering infrastructures.
| Blocklist | Gmail | Outlook / Microsoft 365 | Yahoo Mail | Orange / Wanadoo |
|---|---|---|---|---|
| Spamhaus ZEN | Indirect (reputation) | Direct SMTP rejection | Direct SMTP rejection | Active filtering |
| Spamhaus DBL (domains) | Active filtering | Active filtering | Active filtering | Partial |
| Barracuda BRBL | Not used | Not used | Not used | Not used |
| Microsoft SNDS | No effect | Critical (internal) | No effect | No effect |
| Yahoo Sender Hub | No effect | No effect | Critical (internal, also covers AOL and AT&T) | No effect |
| UCEPROTECT L1 | Ignored | Ignored | Ignored | Ignored |
Gmail primarily operates on its own internal reputation signals. Google Postmaster Tools is its observation interface. A Spamhaus listing can indirectly influence its overall reputation, but Gmail does not block at the SMTP level solely based on external DNSBLs.
SORBS is dead. UCEPROTECT: ignored by ESPs, risky with hosting providers
SORBS closed its doors on June 5, 2024. Proofpoint, which had acquired the service in 2011, decided to shut it down due to an unviable business model. The database is no longer updated, DNS queries return no reliable results, and removal requests are no longer processed. If your monitoring tool still checks it, remove it from your dashboard. You’re looking at phantom data.
UCEPROTECT is a different issue. Three levels, each with their own consequences:
- L1: Individual IP that sent spam. Questionable criteria.
- L2: Entire subnet (/24) if too many L1 IPs are listed. Your data center neighbors can drag you there.
- L3: Entire ISP or hosting provider network. A single abuse at OVH could theoretically list thousands of servers.
UCEPROTECT offers a paid expedited removal service (Whitelisting Express). This practice is documented and well-known in the deliverability community. Gmail, Outlook, and Yahoo ignore it: a UCEPROTECT listing has no effect on deliverability to large public email providers.
However, some hosting providers and network operators use UCEPROTECT as an internal signal. An L2 or L3 listing can trigger a port 25 suspension or service termination from the hosting provider — unrelated to your actual deliverability. So keep an eye on it — not for open rates, but because your host could cut your service at a moment’s notice.
The Microsoft SNDS case: a unique internal list
Microsoft’s Smart Network Data Services (SNDS) is not a blocklist in the traditional sense. It is a dashboard that Microsoft provides to senders to view the reputation of their IPs with Outlook.com and Microsoft 365. No third party can query SNDS via DNS.
Yet, it’s one of the most useful tools for diagnosing deliverability issues with Microsoft recipients. In March 2026, a snapshot of 990,000 analyzed domains showed that 70.9% had no effective DMARC protection. These are the domains that drive poor SNDS scores.
The practical approach: register your IP ranges on postmaster.live.com and regularly check the color of the indicators (green, yellow, red). A red SNDS indicator is a more reliable alert than being listed on 90% of the market’s DNSBLs.
Yahoo Sender Hub: the counterpart for Yahoo, AOL, and AT&T
Since July 2025, Yahoo has also been the gateway for AT&T: att.net, sbcglobal.net, bellsouth.net, pacbell.net, and prodigy.net now route through its infrastructure. This consolidation places three consumer ecosystems — previously managed separately — under a single filter.
The official entry point for diagnosing deliverability issues to Yahoo Mail, AOL Mail, and AT&T domains is Yahoo Sender Hub, available at senders.yahooinc.com. Like Google Postmaster Tools for Gmail or SNDS for Microsoft, it is not a blocklist in the classical sense: it is a dashboard providing aggregated delivery statistics, complaint rates, and DMARC feedback for verified DKIM domains.
Its Insights feature, rolled out in 2025, displays metrics per sending domain. For any infrastructure sending a share of traffic to these three networks, it is now the only direct signal available before the inbox.
How many blocklists should be monitored in practice?
MXToolbox or MultiRBL check up to 200 lists simultaneously. It’s visually reassuring but operationally quite useless. Most of these lists are orphaned, poorly maintained, or without real adoption.
Monitoring 8 to 12 lists covering real vectors is sufficient:
- Spamhaus ZEN, SBL, XBL, DBL (domains)
- SpamCop (SCBL), used by some Cisco filters
- Barracuda BRBL, for B2B sends
- Microsoft SNDS, for traffic to Outlook and Microsoft 365
- Google Postmaster Tools, for domain reputation with Gmail
- Yahoo Sender Hub, for reputation at Yahoo Mail, AOL, and AT&T
The frequency depends on volume. For sends exceeding 10,000 emails per day, hourly checks are recommended. Below that, daily checks are sufficient. Monthly, it’s too late: several campaigns will have been affected between listing and detection.
What to do if listed on a critical blocklist?
Before looking for a delisting form, correct the cause. This is the common rule for Spamhaus, SpamCop, and Barracuda: a request submitted without prior correction comes back within hours. Confirmed spam, purchased list, compromised server — the reason is almost always accessible within the listing details.
“Ending up on a blocklist like Spamhaus can cut emails at major providers. It’s often discovered too late: after open rates collapse, following spikes in complaints, increased bounces, or sending to purchased lists.” — Warmy, Email Blacklist Impact Report, 2026
The delay between listing and metric drop remains the most common blind spot. Regular checks prevent the surprise effect. For list-by-list delisting procedures, see: How to get removed from a blocklist?
FAQ: Email blocklists in 2026
Is SORBS still active in 2026?
No. SORBS was permanently shut down on June 5, 2024, by Proofpoint. It is no longer updated, and DNS queries yield no reliable data. Monitoring SORBS in 2026 is like querying an empty void.
Does Gmail use external blocklists like Spamhaus?
Gmail does not reject at the SMTP level based solely on external DNSBLs. It relies on its own reputation signals, visible via Google Postmaster Tools. A Spamhaus listing can indirectly contribute to a poor overall reputation if it indicates problematic sending behavior.
Is being listed on UCEPROTECT L3 serious?
Not for deliverability to Gmail, Outlook, or Yahoo — these providers ignore UCEPROTECT. However, some hosting providers (OVH, Hetzner…) use it as an internal trigger for port 25 suspension or service termination. An L2 or L3 listing can thus take your infrastructure offline independently of any impact on mailboxes. Monitor it, but not at the same level as Spamhaus.
What is the difference between an IP blocklist and a domain blocklist?
An IP blocklist like Spamhaus ZEN targets the sending server’s IP address. A domain blocklist like Spamhaus DBL targets the domain appearing in the email’s body or headers. A domain can be listed without the IP being listed and vice versa. Both types should be monitored independently.
How can I check if my IP is on a blocklist?
Free tools like MXToolbox Blacklist Check or DNS Checker allow one-off checks on dozens of lists. For continuous monitoring, services like Captain Verify, Validity, or Postmark offer automated alerts.
In 2026, the question is no longer “am I on a blocklist?” but “on which list and is this list consulted by my recipients’ servers?”
