Yahoo / AOL "553 5.7.1 [BL21] Connections will not be accepted ... Spamhaus's list" (BL23): how to fix it

What it means

Yahoo Mail and AOL Mail (both run by Yahoo) check every connecting server against the Spamhaus blocklists. If your sending IP address is listed, they refuse the connection before your email is even sent. The bounce message usually looks like this:

553 5.7.1 [BL21] Connections will not be accepted from 203.0.113.10, because the ip is in Spamhaus's list; see https://...

You may also see a [BL23] code with a similar sentence. Both codes mean the same thing for you: the IP address that sent your email is on a Spamhaus list. The code alone does not reliably tell you which Spamhaus list it is, so you need to look it up.

Spamhaus is a non-profit that publishes several lists:

• SBL: IPs that Spamhaus has seen sending spam or hosting spam operations.
• XBL / CSS: IPs that look infected or hacked (malware, compromised accounts, misconfigured servers sending junk).
• PBL: IP ranges that should not send email directly, such as home internet connections and some cloud ranges. Being on the PBL is not a punishment; it is a policy.

Why it happens

• A hacked mailbox, website or contact form on your server sent spam.
• You send directly from a home or office internet connection (PBL).
• You rent a cloud or VPS IP that a previous customer abused.
• You share an IP with other senders at your hosting company, and one of them sent spam.
• Your server has no proper reverse DNS name (PTR) and looks like a dynamic IP.

How to check

1. Find the IP. Copy the IP address written in the bounce. That is the one Yahoo saw.
2. Look it up at Spamhaus. Go to check.spamhaus.org and enter the IP. The result tells you which list it is on and why.
3. Run a Delivmail test. On the home page (/), copy your unique test address and send it an email from the same server or tool that bounced. The report shows the sending IP, its blacklist results, its reverse DNS and whether it looks like a shared provider or your own server.
4. Check other lists. Use /blacklists to test the IP and your domain on several blocklists at once. A listing elsewhere often points to the same problem.

How to fix it

If the IP is on the SBL

The SBL is managed by hand. The listing usually names the reason. If you use a hosting company or email provider, contact them: they must fix the problem (often a spamming customer) and ask Spamhaus to remove it. If it is your own server, stop the spam source first, then follow the removal steps shown on the Spamhaus page.

If the IP is on the XBL or CSS

• Look for the cause: a hacked email account (change its password), a vulnerable website plugin, a contact form without protection, or malware on a machine behind the same IP.
• Check your mail logs for big bursts of outgoing mail you did not send.
• Once the cause is removed, use the removal option on check.spamhaus.org. If the problem is still there, the IP will be listed again.

If the IP is on the PBL

• If you send from a home or office connection, do not send directly. Send through your provider's outgoing server (Google Workspace, Microsoft 365, your hosting company or a sending service).
• If it is a real mail server with a fixed IP, set a proper reverse DNS name (for example mail.yourdomain.com) and use the self-removal form on the Spamhaus site.

If you use a shared provider

You cannot delist a shared IP yourself. Open a ticket with your provider, paste the full bounce, and ask them to move you to a clean IP or fix the listing.

Check that it is fixed

Look the IP up again on check.spamhaus.org: it should show no listing. Removals usually take effect within minutes to a few hours, but Yahoo may keep its own cached result a little longer. Send a new Delivmail test and confirm the blacklist section is clean, then resend your email to the Yahoo or AOL address that bounced. If you are listed again soon after, the cause is still active: keep looking for the hacked account or script.