When an outgoing email bounces with an alert stating your sending IP address is on a blocklist (or DNSBL/RBL), the recipient’s mail transfer agent (MTA) has rejected your connection based on anti-spam database records. For Google Workspace users, this usually stems from an on-premise SMTP relay IP, an unauthenticated marketing server, or a shared Google pool IP that has triggered temporary automated filters. To fix it, you must identify whether the blocked IP belongs to Google, a third-party relay, or your local network, and take corrective remediation steps.
How to Fix an “IP is on a Blocklist” Email Bounce
Extract the blocked sending IP address and DNSBL name from your bounce message (Non-Delivery Report). If the IP belongs to a private server or custom relay, stop any spam outbreaks, fix missing reverse DNS (rDNS/PTR), and submit a delisting request to the blacklist operator. If it is a shared Google Workspace IP, Google automatically handles mitigation, but you must verify that your SPF, DKIM, and DMARC records are correctly aligned.
What the Blocklist Signal Means
Receiving email servers query public and private Real-time Blackhole Lists (RBLs) and Domain Name System Blocklists (DNSBLs) during the initial SMTP handshake. When your sending IP address matches an entry in a list like Spamhaus, Barracuda, Invaluement, or SORBS, the receiving MTA terminates the session or drops the message before delivery.
This rejection typically generates an enhanced SMTP status code such as 550 5.7.1 or 554 5.7.1. The bounce notice returned to the sender normally specifies:
- The connecting IPv4 or IPv6 address.
- The specific blacklist provider (e.g.,
zen.spamhaus.org,b.barracudacentral.org). - A reference URL or response code explaining why the IP was flagged.
A blocklist event is an external reputation penalty against an IP address, but receiving servers frequently combine this with domain reputation checks. If your domain lacks authentication, a single IP listing can result in rejections across multiple destinations via Resolving “554 5.7.1” Message rejected (SPAM suspicion).
Step 1: Identify Who Owns the Blocked IP
Before attempting remediation, look at the Non-Delivery Report (NDR) to see which IP was actually blocked. The diagnostic path differs completely depending on who owns the address.
To inspect the raw transaction, follow How to Read Gmail “Original Message” Headers and locate the Received: header closest to the rejection event.
Incoming Bounce Header
│
▼
Extract Blocked IP Address
│
┌─────┴────────────────────────┐
▼ ▼
Google-Owned IP Range Dedicated / Custom IP Range
(e.g., 209.85.x.x) (Local MTA, VPS, Marketing Relay)
│ │
▼ ▼
Shared Pool Architecture Private Infrastructure Audit
• Do not request delisting • Identify outbound spam source
• Verify SPF, DKIM, DMARC • Check reverse DNS (rDNS/PTR)
• Monitor Workspace health • Submit manual delisting form
Scenario A: The Blocked IP Belongs to Google (209.85.x.x or similar)
Google Workspace routes outbound traffic through large, rotating pools of shared IP addresses. Occasionally, another tenant on the same pool sends spam, landing that specific Google IP on an aggressive RBL (such as UCEPROTECT Level 2/3 or SORBS).
- What you should NOT do: Do not contact the blacklist provider. Blacklist operators will not honor delisting requests from individual domain tenants for IPs owned by Google LLC.
- What you should DO: Major providers (like Microsoft 365, Yahoo, and corporate gateways) generally ignore minor, abusive RBLs for large shared providers like Google. If a major provider does reject your mail based on a Google IP, ensure your domain-level authentication is pristine. Strong cryptographic signatures allow receiving filters to trust your domain even when the shared IP reputation dips temporarily. Check your setup against The Workspace Email Authentication Master Checklist.
Scenario B: The Blocked IP Belongs to Your Custom SMTP Relay or Server
If the blocked address points to your office static IP, an on-premise Exchange/Postfix server, or a cloud VPS used for transactional mail:
- You are directly responsible for the reputation of that IP.
- You must clean your mail flow and request delisting yourself.
Step 2: Audit Custom Infrastructure for Compromise
If your dedicated IP appears on an RBL, requesting a delisting without fixing the underlying leak will result in an immediate re-listing and permanent reputation loss. Complete these three internal checks first:
Check 1: Audit Outbound Mail Queues for Spam Outbreaks
Review your mail server logs for unauthorized connections, cracked user credentials, or compromised web forms:
- Check for high outbound message volume originating from a single user account or script.
- Ensure your relay is not configured as an open relay. Verify that authentication is mandatory for all outbound traffic.
Check 2: Verify Reverse DNS (PTR) Records
Many blocklists automatically list IPs that lack a valid Pointer (PTR) record or have generic ISP hostnames (e.g., 123-45-67-89.isp.net).
- The IP address must resolve backwards to a Fully Qualified Domain Name (FQDN).
- That FQDN must resolve forward to the exact same IP address (Forward-Confirmed Reverse DNS).
- If your PTR is broken, fix it with your ISP or hosting provider before applying for delisting. For guidance, see “550 5.7.25” IP Address Reverse DNS (rDNS) Failure and Resolving “PTR Record Missing” for SMTP Relay.
Check 3: Check Forward Confirmed DNS Setup
Run a quick query from your terminal to verify your hostname alignment:
host 198.51.100.25
# Should return: mail.yourdomain.com
host mail.yourdomain.com
# Must return: 198.51.100.25
Step 3: Delisting Procedures for Major DNSBLs
Once your infrastructure is secured and PTR records match, navigate to the specific blacklist provider mentioned in your bounce report:
- Spamhaus (SBL, XBL, PBL, ZEN): Visit the Spamhaus IP and Domain Reputation Checker. Search your IP. If listed in the SBL (Spamhaus Block List) or XBL (Exploits Block List), read the exact case log provided. Once the infected host or vulnerability is eliminated, follow their removal form instructions.
- Barracuda (BRBL): Navigate to the Barracuda Central reputation lookup. Enter your IP and email address, along with an explanation of the corrective action taken to secure the server.
- SpamCop: SpamCop relies on automated reporting traps. Listings expire automatically 24 to 48 hours after the spam traffic completely stops. Manual delisting is rarely required.
- SORBS / UCEPROTECT: Many modern corporate mail administrators treat these lists with skepticism due to aggressive listing practices across broad IP subnets. Focus on your SPF, DKIM, and DMARC alignment rather than paying any fees for expedited removal.
What to Do When Domain Reputation Takes the Hit
If your IP was blacklisted for an extended period, your sending domain’s reputation likely dropped alongside it. Even after an IP is removed from a DNSBL, receiving systems like Gmail may continue sending your mail to the spam folder or issuing “5.7.26” Email blocked due to low reputation.
To repair this damage, register your domain in Google Postmaster Tools to track your Domain Reputation score, reduce sending volumes temporarily, and ensure your message list hygiene is strictly maintained. For complete steps on recovering your standing, consult Resolving “Message blocked due to domain reputation”.