How do Mimecast and Proofpoint scrutinize senders, and what best practices can improve inbox placement beyond whitelisting?
Michael Ko
Co-founder & CEO, Suped
Published 13 May 2025
Updated 17 Aug 2025
6 min read
Email deliverability to B2B recipients can be particularly challenging, often due to stringent email security solutions like Mimecast and Proofpoint. These platforms are designed to protect corporate inboxes from a wide array of threats, from spam to sophisticated phishing attacks. While their primary goal is security, their filtering mechanisms can inadvertently impact legitimate business communications, sometimes diverting them to quarantine or even blocking them outright.
Understanding how these systems scrutinize incoming mail is crucial for improving inbox placement. It goes beyond simply asking recipients to whitelist your email address, which is often a temporary or impractical solution for scale. Instead, a comprehensive approach focusing on email best practices is essential.
My goal here is to delve into the common filtering criteria used by Mimecast and Proofpoint, and then outline effective strategies that can significantly enhance your email deliverability, ensuring your messages land in the inbox, not the junk folder or quarantine.
Understanding Mimecast and Proofpoint filtering
Mimecast and Proofpoint are leading enterprise email security solutions, and while they share the common goal of protecting inboxes, their specific methodologies and strengths can vary. Generally, they scrutinize senders based on a combination of technical factors, sender reputation, and content analysis.
One key area is the assessment of sender IP address reputation. Mimecast, for example, is known to leverage various external blacklists (or blocklists), including SpamCop, which can be particularly challenging for senders using shared IP addresses. If another sender on the same shared IP engages in poor sending practices, your legitimate emails could be affected, leading to deliverability issues.
Proofpoint also heavily relies on IP reputation, alongside domain reputation and email authentication results. They perform deep content analysis, looking for malicious URLs, attachments, and suspicious patterns that indicate phishing or spam. Both solutions also factor in user feedback, meaning if a significant number of recipients mark your emails as spam, your sender reputation will suffer, leading to more aggressive filtering.
Scrutiny Area
Mimecast Focus
Proofpoint Focus
IP Reputation
Heavy reliance on external blacklists (e.g., SpamCop).
Strong emphasis on IP reputation alongside domain reputation.
Domain Reputation
Evaluates domain history, authentication, and user complaints.
Crucial for identifying trusted sources and mitigating spoofing.
Content Analysis
Scans for keywords, suspicious links, attachments, and spam patterns.
Advanced threat detection, URL rewriting, and attachment sandboxing.
Email Authentication
Verifies SPF, DKIM, and DMARC records to prevent spoofing.
Leverages authentication results to validate sender legitimacy.
User Feedback
Incorporates recipient spam complaints into sender reputation scores.
Tracks user reported spam to adjust filtering sensitivity.
Core deliverability factors
Beyond whitelisting, improving inbox placement requires a multi-faceted strategy focused on maintaining a strong sender reputation and adhering to email best practices. This is similar to how major email providers approach security, by scrutinizing a wide range of signals.
One fundamental step is to move away from shared IP addresses for your promotional or bulk email traffic. Shared IPs can be a deliverability nightmare, as your sender reputation becomes tied to the practices of other users on that IP. A dedicated IP address, especially when properly warmed up, gives you full control over your sending reputation.
Additionally, segmenting your email traffic by type and using separate subdomains can significantly enhance deliverability. For instance, transactional emails (like order confirmations) should be sent from a different subdomain and potentially a different IP than marketing newsletters. This isolation prevents issues with one stream from affecting the deliverability of another, often more critical, stream.
Best practices for email deliverability
Dedicated IP addresses: Establish your own sending reputation for better control. This is especially important for B2B sending as corporate firewalls are more sensitive.
Subdomain segmentation: Use distinct subdomains for different email types, such as marketing.yourdomain.com and transactional.yourdomain.com.
IP warmup: Gradually increase sending volume on new IPs or subdomains to build a positive sending history.
List hygiene: Regularly clean your email lists to remove inactive, invalid, or problematic addresses.
Consent management: Ensure all recipients have explicitly opted in to receive your emails. This is critical for B2B as unsolicited emails are often flagged by security solutions.
Technical authentication and infrastructure
Robust email authentication is non-negotiable for improving inbox placement, particularly with sophisticated security gateways like Mimecast and Proofpoint. SPF, DKIM, and DMARC are crucial for verifying sender identity and protecting against spoofing. These security protocols help receiving servers, including those protected by corporate solutions, trust that your emails are legitimate and have not been tampered with.
Specifically, an accurate and properly configured DMARC record with an enforcing policy (p=quarantine or p=reject) signals to Mimecast and Proofpoint that you are serious about email security. It provides instructions on how to handle emails that fail authentication, significantly reducing the chances of your legitimate emails being mistaken for phishing attempts. Transitioning your DMARC policy needs to be done carefully to avoid unintended deliverability issues.
Beyond the basic setup, ensuring perfect alignment between your sending domain and your authentication records is key. Mimecast and Proofpoint are sophisticated enough to detect subtle misconfigurations that might pass basic checks but still trigger their advanced filters. Regularly monitoring your DMARC reports is essential to catch and fix any authentication failures promptly.
Beyond technical configurations, the content of your emails and how recipients interact with them play a significant role. Generic, sales-heavy content with numerous links or attachments can trigger spam filters. Personalization, clear calls to action, and relevant information are far more effective.
Engagement metrics are also closely watched. If recipients consistently open, click, and reply to your emails, it sends positive signals to security solutions. Conversely, low open rates, high bounce rates, and frequent spam complaints will quickly tank your sender reputation, making it harder to reach the inbox, even if your technical setup is perfect.
Regularly monitoring your blocklist (blacklist) status is critical. Getting listed on a major blocklist can severely impact your deliverability to Mimecast and Proofpoint users. When issues arise, prompt identification and rehabilitation of your sender reputation are key to restoring healthy deliverability.
Content and engagement
Email content should be highly relevant and personalized to the recipient, especially in B2B contexts. Avoid generic templates, excessive images, or spammy keywords that could trigger filters. Focus on providing value rather than solely selling. High engagement rates (opens, clicks, replies) are positive signals, while low engagement and high complaints are red flags.
Monitoring and remediation
Continuously monitor your sender reputation and blocklist status. Implement DMARC reporting to gain visibility into authentication failures. Address any issues promptly, such as removing unengaged subscribers or fixing technical misconfigurations. Proactive monitoring helps you stay ahead of potential deliverability problems.
Views from the trenches
Best practices
Maintain separate sending infrastructure for transactional and marketing emails using dedicated IPs.
Always enforce robust email authentication (SPF, DKIM, DMARC) with a strict DMARC policy.
Segment your audience and tailor content to ensure high engagement and relevance.
Regularly clean your email lists to remove invalid or unengaged contacts.
Actively monitor your sender reputation and blocklist status to catch issues early.
Common pitfalls
Sending B2B emails from shared IP addresses, increasing blocklist risk.
Lacking a DMARC policy or having one set to p=none, failing to enforce authentication.
Sending irrelevant or overly promotional content to B2B audiences.
Not regularly cleaning email lists, leading to high bounce rates and spam complaints.
Ignoring DMARC reports, missing critical insights into email authentication failures.
Expert tips
Prioritize dedicated IP addresses for B2B sending as corporate filters are highly sensitive.
Ensure proper DMARC implementation and gradually move to an enforcing policy (quarantine/reject).
Focus on quality over quantity for B2B email lists to maximize engagement and avoid spam traps.
Actively encourage recipients to add you to their address book or safe sender list.
Monitor your delivery rates to specific domains protected by Mimecast/Proofpoint.
Expert view
Expert from Email Geeks says Mimecast utilizes SpamCop, which is particularly challenging for shared IP addresses, making dedicated IPs crucial for B2B deliverability.
2023-12-13 - Email Geeks
Expert view
Expert from Email Geeks says Proofpoint likely considers IP reputation as a significant factor in its filtering decisions, similar to other major security gateways.
2023-12-13 - Email Geeks
Beyond the whitelist: mastering deliverability
Achieving consistent inbox placement with organizations using Mimecast and Proofpoint requires a proactive and comprehensive approach that extends far beyond simple whitelisting. It's about building and maintaining a strong sender reputation through meticulous adherence to email best practices.
By investing in dedicated IPs, segmenting your sending, implementing robust authentication, and focusing on high-quality, engaging content, you can significantly improve your chances of landing in the inbox. This continuous effort in deliverability best practices will ultimately lead to more successful email campaigns and better engagement with your B2B audience.
Heavy reliance on external blacklists (e.g., SpamCop).
Strong emphasis on IP reputation alongside domain reputation.
