Mail that was delivering fine suddenly starts bouncing, or a receiving server’s rejection message mentions a domain like zen.spamhaus.org or b.barracudacentral.org. That’s a DNS-based blocklist (DNSBL) — a real-time, queryable list of IP addresses reported for sending spam, running an open relay, or being part of a botnet. Almost every one of these has a free, self-service way to get removed once the underlying cause is actually fixed.

Step 1: Confirm You’re Actually Listed

Query the blocklist directly rather than trusting a bounce message alone — the format reverses your IP octets and appends the DNSBL’s zone:

# Checking if 198.51.100.7 is listed on Spamhaus ZEN
dig +short 7.100.51.198.zen.spamhaus.org

# Any A record response (e.g. 127.0.0.4) means it's listed;
# no response at all means it's clean on that particular list

Run the same reversed-octet query against each blocklist mentioned in your bounce logs, since a single incident can land you on more than one simultaneously.

Step 2: Find the Root Cause Before You Do Anything Else

Common cause	How it usually happens
Compromised WordPress or CMS site	A plugin vulnerability lets an attacker send spam through the server’s mail() function
Open relay misconfiguration	Mail server accepts and forwards mail for domains it shouldn’t
Compromised email account	A weak or reused password lets an attacker send spam through a real mailbox
Purchased or scraped recipient list	High complaint rate reported directly to the blocklist operator
Shared IP, another tenant’s abuse	On shared hosting, someone else on the same IP triggered the listing

Step 3: Fix It Before You Request Removal

Requesting delisting before the cause is resolved almost always fails review, and on some lists it burns a limited number of removal attempts. Work through this before submitting anything:

# Check the mail queue for a spam flood
exiqgrep -f 'noreply@' | wc -l    # Exim
mailq | grep -c '^[A-Z0-9]'        # Postfix

# Clear a confirmed spam flood
exiqgrep -f 'attacker@compromised-domain.com' -i | xargs exim -Mrm   # Exim

Change every credential that could have been compromised, patch or remove the vulnerable plugin/script, and confirm the outbound spam has actually stopped — not just slowed down — before moving on.

Step 4: Request Removal

Blocklist	How to request removal
Spamhaus (SBL/CSS/XBL)	Their public removal lookup and form at their website, per listed IP; explain the remediation performed
Barracuda Reputation Block List (BRBL)	Barracuda’s public reputation lookup page has a self-service removal request
SORBS and most smaller DNSBLs	Web-based removal form, often instant for automatically-expiring listings

How Long It Takes

Automatically-maintained lists (many exploit/botnet-based ones) often clear on their own within hours once the abusive traffic stops, since they continuously re-test listed IPs. Manually reviewed lists like Spamhaus’s SBL can take longer because a person verifies the remediation before approving removal — padding your removal request with specifics (what was compromised, what was patched, when the spam stopped) speeds that review up considerably.

Preventing Re-Listing

Preventive measure	Protects against
Disable PHP’s mail() where not needed, route through authenticated SMTP	Compromised scripts spamming silently
Keep CMS, plugins and mail software patched	Known-vulnerability exploitation
Rate-limit outbound mail per account/script	Runaway sends before you notice
Monitor mail queue size and set alerts	Catching a flood in minutes instead of days

Conclusion

A blocklist listing is a symptom, not the disease — fix the compromise or misconfiguration that caused it first, confirm the abusive traffic has genuinely stopped, and only then submit a removal request with the specifics of what you fixed. Skip straight to the removal form and you’ll likely be back on the list within days.