Why IPs Get Blacklisted
WARNING
Bouncify doesn't provide any guarantee that your host IP will not be blacklisted. When sending bulk email verification requests to external servers through SMTP, there's always a risk of blacklisting. Even though Bouncify uses custom rules and filtering without sending actual emails to third-party mailboxes, mail servers with strict anti-spam filtering may flag your verification activities as suspicious, potentially leading to blacklisting. This guide explains how blacklisting happens and how it can be prevented, though not completely eliminated.
Why Does Blacklisting Happen?
Blacklisting is triggered by various criteria set by email service providers. When a certain threshold of suspicious activity is detected from a particular IP address within a specific timeframe, providers like Google or Microsoft will flag and blacklist the sending IP address. Once blacklisted, emails sent from that IP will bounce back.
Common triggers include:
- High volumes of SMTP connections to check email validity
- Hitting spam traps
- Suspicious connection patterns
- Verification attempts on non-existent email addresses
- Lack of proper email authentication
This is particularly common with mail servers protected by firewalls and advanced anti-spam security systems, such as Google, Microsoft, Yahoo, Mimecast, and Comcast. For some popular mail servers, Bouncify uses alternative verification methods that don't rely on direct SMTP connections.
As spam detection technology advances, email providers become increasingly protective of their systems and customer information. Each provider follows different patterns to prevent unauthorized access and reports any suspicious activity.
IPs without proper rDNS PTR records and missing DKIM/SPF configurations for their FQDN are more likely to be blacklisted due to improper SMTP configuration. When using such configurations to connect to external servers, your requests may be denied, and your IP reported to spam database lists.
New IP addresses are particularly vulnerable. When you begin using a new server for sending bulk SMTP connections to verify email addresses, strict security systems may interpret this as suspicious activity, especially when checking non-existent addresses. This can result in your IP being reported to DNSBL (Domain Name System Blacklist) service providers. Once blacklisted, your IP reputation is compromised, requiring significant effort to restore.
How to Prevent Blacklisting
While we cannot guarantee complete prevention of blacklisting (as it's beyond our control), following these practices can significantly reduce the risk:
Monitor Your Blacklist Status: Set up your IP and domain with a free blacklist monitoring system to receive notifications whenever your host IP or domain gets blacklisted.
Ensure Proper Email Server Configuration: Make sure your SMTP mail server setup passes mail-tester.com checks with a high score (aim for 10/10).
Age Your Domain: New domains should wait 7-14 days (or as recommended by mail-tester.com or DNSBL servers) before conducting bulk email verification processes.
Use FQDN for SMTP Server: Configure your SMTP server with a Fully Qualified Domain Name (e.g., mail.yourdomain.com, vps.yourdomain.com).
Align Hostname and rDNS: Ensure your hostname contains the same rDNS PTR record domain and points to the same IP.
Configure Port 25: Make sure SMTP server port 25 is open and can communicate with third-party SMTP servers.
Implement Authentication: Set up proper DKIM, SPF, rDNS, and PTR records for your server.
Warm Up Your IP: If using a new host IP, gradually increase the volume of verification requests to build trust with DNSBL networks:
- Start with 100 verifications per day
- Increase to 1,000 per week
- Then 100,000 per month
- Eventually 100,000 per day
Older IPs with good reputations are less likely to be blacklisted and are delisted more quickly if issues occur. If you continue sending verification requests without fixing IP reputation issues, your IP will quickly be blacklisted again.
Use One Instance per Server: Avoid running multiple Bouncify instances from a single VPS. Always use the latest version of Bouncify to benefit from important feature updates.
Use Legitimate Configurations: Don't use fake emails, public domain emails, or questionable SMTP configurations in your system settings.
What to Do If Blacklisted
If your IP gets blacklisted despite following preventive measures:
Identify the Blacklist: Use tools like MXToolbox to determine which blacklist you're on.
Review Your Sending Practices: Identify and correct the behavior that led to blacklisting.
Request Delisting: Most blacklist providers have a process to request removal once issues are fixed.
Consider IP Rotation: For persistent issues, you may ask your VPS provider to replace your host IP for a small fee and reconfigure DNS records.
Implement Rate Limiting: Use Bouncify's built-in rate limiting features to control verification request volumes.
Remember that DNSBL networks are particularly strict with new IPs. If blacklisted for the first time, reduce your verification volume and build reputation gradually rather than continuing at high volumes.
Conclusion
While the risk of IP blacklisting cannot be eliminated entirely when performing email verification, following best practices significantly reduces this risk. Bouncify continues to improve its system architecture to help users reduce blacklisting issues and maintain better control over data responses. However, preventing blacklisting ultimately remains the user's responsibility.