What steps can be taken to resolve widespread email blocking and spam issues for B2B opt-in emails?
Michael Ko
Co-founder & CEO, Suped
Published 1 Jul 2025
Updated 16 Aug 2025
11 min read
Dealing with widespread email blocking and spam issues for B2B opt-in emails can be incredibly frustrating. You’ve put in the work to get legitimate opt-ins, yet your messages aren't reaching their intended inboxes. It often feels like you're caught in a complex web of technical configurations, sender reputation challenges, and ever-evolving spam filters. This is especially true when sending to large corporate domains like Cisco or Capital One, which employ sophisticated security measures, often relying on specific email security vendors or their own proprietary systems.
The key to resolving these issues isn't a single fix, but rather a holistic approach that addresses various aspects of your email program. It requires diligent monitoring, adherence to best practices, and a proactive stance on maintaining a strong sender reputation. Let's explore the steps you can take to diagnose and overcome these challenges, ensuring your valuable B2B communications land where they belong: in the inbox.
Diagnosing the core issue
The first step in tackling email blocking is to understand why your emails are being rejected or filtered to spam. This often starts with analyzing the bounce messages you receive. A simple 'Unknown user' bounce, for instance, tells you immediately that the recipient address does not exist, and continuing to send to it will harm your sender reputation significantly. These clear bounce codes are crucial for diagnosing the problem at its source.
However, sometimes bounce messages are less descriptive, or your sending software might not provide the full error code. In such cases, you need to dig deeper into the recipient's email infrastructure. Large B2B organizations often use specific email security gateways. For example, some companies might use Cisco's own internal spam filters, while others, like Capital One, are known to be Proofpoint users. Identifying these systems can help you tailor your outreach and understand specific requirements for whitelisting or delisting.
Your sending reputation is a critical factor. Internet Service Providers (ISPs) and email security gateways (like Proofpoint) use a sender's reputation to determine whether to deliver an email to the inbox, quarantine it, or block it entirely. A poor reputation, often caused by high bounce rates, spam complaints, or sending to stale lists, will lead to widespread blocking.
It's important to differentiate between hard bounces (permanent delivery failures) and soft bounces (temporary issues). Hard bounces, such as 'unknown user,' indicate a non-existent email address and should be removed from your list immediately. Continual sending to hard bounces signals poor list hygiene and can severely damage your sender reputation, increasing your chances of being placed on a blacklist or blocklist. Understanding these distinctions is fundamental to why your emails are going to spam.
Technical authentication and infrastructure
A robust technical setup is the foundation of good email deliverability. This includes correctly configuring and maintaining your email authentication protocols: Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC).
SPF: SPF specifies which mail servers are authorized to send email on behalf of your domain. An incorrect SPF record can lead to legitimate emails being flagged as spam or outright rejected. Ensure your SPF record includes all IP addresses and domains from which you send email, including any third-party Email Service Providers (ESPs). You can learn more about what SPF is and how it works.
DKIM: DKIM adds a digital signature to your emails, allowing recipient servers to verify that the email has not been tampered with in transit and truly originated from your domain. A missing or invalid DKIM signature can significantly reduce your deliverability. If you see DKIM record not found errors, address them immediately.
DMARC: DMARC builds upon SPF and DKIM, providing instructions to recipient servers on how to handle emails that fail authentication. It also provides valuable aggregate and forensic reports that tell you exactly who is sending email on your behalf and whether they're passing authentication. Implementing DMARC with a policy of p=none initially allows you to monitor your email flow without affecting deliverability, then gradually move to quarantine or reject as you gain confidence.
These authentication protocols are essential for establishing trust with receiving mail servers, especially for B2B communications where security is paramount. Neglecting any of them can lead to significant deliverability issues, even if your list is perfectly opt-in. Many ISPs, including Gmail and Microsoft, now require strict adherence to these standards.
Beyond the initial setup, consistent monitoring of your DMARC reports is vital. These reports provide insights into authentication failures, sources sending email on your behalf, and potential malicious activity. By regularly reviewing them, you can proactively identify issues that could lead to blocking, such as DKIM temporary errors or unauthorized senders impersonating your domain.
Sender reputation and list hygiene
Even with perfectly configured authentication, a poor sender reputation can torpedo your B2B email deliverability. This often stems from poor list hygiene and inconsistent sending practices. Regularly cleaning your email list is non-negotiable, especially for B2B opt-in lists which can decay quickly as employees change roles or companies. Remove hard bounces immediately, and consider sunsetting unengaged subscribers.
Sending volume and frequency also play a significant role. If you have a dedicated IP address but send emails in infrequent, large bursts (e.g., 50,000 contacts once a month with little to no daily traffic), this erratic pattern can trigger spam filters. ISPs prefer consistent, measured sending volumes. A sudden spike can look like spam activity, even if your emails are legitimate. For new IPs or significant volume changes, a proper IP warm-up strategy is crucial to build a positive sending history.
Spam traps are another serious threat to your sender reputation. These are email addresses specifically designed to catch spammers. Sending to a spam trap, even once, can get your IP or domain placed on a blocklist (or blacklist). The best defense against spam traps is rigorous list cleaning and using a double opt-in process for all new subscribers. This ensures that only genuinely interested recipients are added to your list, significantly reducing the risk of hitting a trap. You can gain a deeper understanding of what spam traps are.
Monitoring your sender reputation through tools like Google Postmaster Tools for Gmail domains or Microsoft's Sender Score is crucial. These platforms provide insights into your spam rate, IP reputation, and domain reputation, allowing you to proactively address issues. Regularly checking for blocklist (or blacklist) listings is also essential. If you find your IP or domain on a blocklist, follow the delisting procedures outlined by the specific blocklist operator. Our article on how to resolve domain blocklisting provides a detailed guide.
Content, engagement, and compliance
Even with perfect technical setup and a pristine list, your email content can still trigger spam filters. Spam filters analyze various elements, including subject lines, body text, links, and attachments, for patterns indicative of spam. Avoiding spam trigger words and suspicious formatting is crucial.
Your engagement metrics are also a powerful signal to ISPs. High open rates, click-through rates, and replies indicate that your emails are valuable and desired, boosting your sender reputation. Conversely, low engagement, high deletion rates without opening, or high spam complaint rates can quickly flag your emails as unwanted, leading to blocking. Segmenting your audience and sending highly relevant content can significantly improve these metrics.
Encourage your B2B recipients to whitelist your sending domain or email address. This is one of the most effective ways to ensure consistent inbox delivery, as it tells their mail server to always trust your emails. Include clear instructions in your welcome emails or on your website. Another key factor is providing a clear and easy way for recipients to unsubscribe, which is a requirement of the CAN-SPAM Act for commercial emails. This reduces the likelihood of recipients marking your email as spam.
Finally, regularly test your email deliverability. Use an email deliverability tester before major sends to identify any potential spam triggers in your content or configuration issues that might impact inbox placement. This proactive testing can help you catch and fix problems before they escalate into widespread blocking.
Advanced strategies and ongoing maintenance
For B2B opt-in emails facing widespread blocking, implementing Brand Indicators for Message Identification (BIMI) can further enhance your brand's legitimacy. BIMI allows your brand's logo to appear next to your emails in supporting inboxes, providing a visual cue of authenticity that can improve trust and engagement. This works in tandem with strong DMARC enforcement.
Consider your IP strategy. While a dedicated IP offers more control over your reputation, it also means you bear full responsibility for that reputation. If you're sending inconsistently or have periods of low volume, a shared IP from a reputable provider might offer better protection against widespread blocking by leveraging the collective positive reputation of other senders. However, for high-volume, consistent sends, a dedicated IP is typically preferred.
Finally, remember that deliverability is an ongoing process. It's not a set-it-and-forget-it task. Regular monitoring, adapting to new ISP requirements, and continuously optimizing your sending practices are essential for long-term inbox success. Staying informed about changes from major mailbox providers, like Outlook's new sender requirements or Google's ongoing updates, will help you maintain optimal deliverability.
By diligently addressing each of these areas, you can significantly improve your B2B opt-in email deliverability and ensure your messages reach their intended recipients, fostering stronger client relationships and driving business growth.
Views from the trenches
Best practices
Maintain a rigorous list cleaning schedule, immediately removing any 'unknown user' or hard bounce addresses.
Implement double opt-in for all new subscribers to ensure high quality and genuine interest.
Ensure your SPF, DKIM, and DMARC records are correctly configured and monitored.
Send email volume consistently, avoiding large, infrequent spikes, especially with dedicated IPs.
Monitor your sender reputation regularly using available postmaster tools.
Common pitfalls
Ignoring 'unknown user' bounce messages and continuing to send to invalid addresses.
Failing to implement or correctly configure DMARC, SPF, and DKIM authentication.
Sending large, inconsistent email volumes from a dedicated IP, which can harm its reputation.
Neglecting to monitor sender reputation or check for blocklist (blacklist) listings regularly.
Using generic or spammy subject lines and content that triggers spam filters.
Expert tips
Analyze MX records of target domains to identify specific email security gateways like Proofpoint or Cisco.
For very specific blocking issues, sometimes the only solution is direct engagement with the recipient organization's IT.
Understand that some ISPs don't strictly follow RFCs for bounce codes, requiring deeper investigation.
A `451 Permanent failure` bounce still means you should stop sending to that address.
Content and email code issues can still lead to spam filtering even with a good sending infrastructure.
Expert view
Expert from Email Geeks says there is no general answer for widespread blocking, so hiring a consultant for specific guidance and analyzing rejection messages is always the initial best step.
2021-12-02 - Email Geeks
Marketer view
Marketer from Email Geeks says examining the MX records of the domains receiving emails can help identify security providers like Proofpoint.
2021-12-02 - Email Geeks
The path to sustained inbox delivery
Resolving widespread email blocking and spam issues for B2B opt-in emails is a complex but manageable challenge. It demands a multi-faceted strategy that encompasses robust technical configurations, diligent sender reputation management, and thoughtful content creation.
By prioritizing accurate email authentication (SPF, DKIM, DMARC), maintaining a clean and engaged email list, and ensuring consistent sending volumes, you lay a strong foundation for deliverability. Furthermore, understanding the nuances of content filtering and actively monitoring your sender reputation will empower you to identify and mitigate problems before they escalate. Adopting these comprehensive steps will help your legitimate B2B communications consistently reach their intended audience, fostering trust and improving your overall email program's effectiveness.
Cisco's own internal spam filters, while others, like 
Capital One, are known to be Proofpoint users. Identifying these systems can help you tailor your outreach and understand specific requirements for whitelisting or delisting.
Microsoft, now require strict adherence to these standards.
