The hype and truth about email blocklists

Understanding Blocklists and Their Impact on Email Deliverability

There are many blocklist organizations that may blocklist your domain or IP address when they believe you’re sending spam. However, only a few blocklists are reputable or have real impact on deliverability.

It’s important to note that anyone can create a blocklist—but a blocklist only matters if mailbox providers actually use it. This article explains what blocklists are, how they work, and which ones you should pay attention to.

What is a blocklist?

A blocklist is a list of domains or IP addresses that have been flagged for sending spam. These lists are maintained and updated in real time by blocklist organizations.

In the early days of email, mailbox providers relied on third-party blocklists to help protect users from spam. Providers like Microsoft would pay for blocklist data and block any emails from senders appearing on those lists.

How blocklists work

Senders are added to a blocklist when they send to spam traps—email addresses that never opted in to receive messages or that have been inactive for years. Sending to spam traps usually means:

The sender isn’t using opt-in data, or
Their list is outdated.

Blocklist organizations place spam traps in locations where spammers typically collect addresses, such as rented lists, scraped web pages, or old inactive inboxes. Anyone who sends to these addresses may be added to a blocklist.

What has changed

Today, major mailbox providers like Gmail, Microsoft, and Yahoo no longer rely on public blocklists. Instead, they use machine learning models to identify spam more accurately.

As a result, most public blocklists have little to no impact on deliverability. Many are outdated or abandoned and are no longer used by any major provider.

How to check if you’re blocklisted

Most blocklists are public, and you can use free tools like MXToolbox to see if your domain or IP is listed.

MXToolbox checks about 100 blocklists, but 95% of them can safely be ignored.
If a recipient’s mail server blocks your message solely because you’re listed on a public blocklist, that’s a sign they’re using outdated security practices.

Note: Some reputable blocklists, like Proofpoint (Cloudmark), do not appear in MXToolbox results. You’ll need to check their websites directly.

Blocklists that actually matter

Only a few blocklists have meaningful impact on deliverability.

Spamhaus

Severity: Very high
Impact: Major mailbox providers respect Spamhaus listings.
Why it matters: Spamhaus operates an advanced network of spam traps to identify senders who send unsolicited messages.
Action: Stop sending immediately, review your practices, and request delisting.
- Check and delist at Spamhaus

Proofpoint (Cloudmark)

Used by: Apple (iCloud.com, mac.com), Cox, Charter, Shaw
Impact: Being listed can block delivery to Apple and several ISP inboxes.
Action: Submit a delisting request.
- Proofpoint (Cloudmark) Delisting Form
- Proofpoint (Apple) Delisting Form

Invaluement

Used by: BTinternet (UK), Mimecast, GMX/Web.de (Germany)
Focus: B2B spam and ESP IP ranges.
Action:
- Invaluement Delisting Form

Barracuda

Type: Security appliance used by corporations.
Impact: Limited to organizations using Barracuda filters.
Action:
- Barracuda Delisting Form

SURBL

Focus: Links and domains within email content.
Action:
- SURBL Delisting Form

Blocklists from specific mailbox providers

Major providers like Gmail, Yahoo, and Microsoft do not use public blocklists. Instead, they rely on internal systems:

Gmail: Use Google Postmaster Tools to check IP/domain reputation.
Microsoft: Check the Smart Network Data Services (SNDS) portal for block information.

These are not blocklists and cannot be delisted manually. They indicate a reputation problem rather than a formal listing.

Smaller providers such as Cox or Comcast may use temporary, private blocks that usually resolve within 24 hours.

What to do if you’re on a blocklist

Blocklists generally don’t affect your deliverability unless they’re one of the reputable ones listed above.

If you are listed:

Submit a delisting request using the relevant form above.
Address the root cause—otherwise, you’ll be re-listed.

Common causes include:

1. Old or stale contact data

Remove contacts who haven’t opened your emails in 6–12 months.
Old addresses may have turned into spam traps.

2. Non–opt-in email addresses

Only send to contacts who explicitly opted in.
Avoid purchased or traded lists.

3. Spam bot attacks on forms

Bots can fill your forms with fake addresses.
Add CAPTCHA and use double opt-in to prevent this.

4. Typo or invalid addresses

Misspelled domains (e.g., “gmal.com”) can trigger spam traps.
Validate addresses and use confirmed opt-ins where possible.

Key takeaway

If you follow email best practices—sending to opt-in, engaged recipients and maintaining clean data—you should rarely encounter a blocklist issue.

If a recipient blocks your messages based on an obscure or outdated blocklist, it likely reflects their internal policy rather than your reputation. In those cases, you can request to be whitelisted with that recipient.