Why Do My Cold Emails Get Blocked by Mimecast, Proofpoint, or Other Security Filters?

If you are sending cold emails, you may notice that some never reach inboxes at companies that use Mimecast, Proofpoint, Barracuda, or similar secure email gateways (SEGs). This is common. Many teams assume it can be fixed with the right settings or tool. In reality, these systems are designed to stop unsolicited outreach.

The Reality About Enterprise Filters

SEGs are built to block cold outreach. Their purpose is to protect employees from unsolicited messages.

They analyze more than domain reputation. Common signals include:

• Sending patterns and volume
• Message fingerprints and content style
• Links, URLs, and headers
• Known cold email infrastructure
• Once traffic is identified as cold outreach, SEGs will block or quarantine it by design.

Why This Is Not “Fixable”

• Domain warming and minor technical tweaks do not change the policy of the receiving company.
• Even teams with excellent infrastructure and practices are filtered when emailing SEG‑protected domains.
• This is not a SuperSend bug. It is a deliberate security choice by those organizations.

What You Can Do Instead

• Target where email works best. Focus on SMB and mid‑market companies that use Gmail, Outlook, or lighter filters.
• Use multi‑channel outreach. Add LinkedIn, phone, events, and other touchpoints for enterprise accounts. Email alone rarely breaks through SEGs.
• Set realistic expectations. Lower reply rates are normal when you target companies behind Mimecast or Proofpoint.

How SuperSend Helps

• Improves deliverability where it is realistically possible.
• Provides multi‑channel capacity so email is not your only lever.
• Offers safe scaling: domains, warmup, and sending infrastructure. No platform can override enterprise security systems, and we will never claim to do so.

Key Takeaway

If your emails are consistently blocked by Mimecast, Proofpoint, or similar filters, this is expected behavior. It is not a solvable technical problem. The right approach is to combine email with other channels and prioritize markets where email remains effective.

Need help planning a multi‑channel strategy? Contact our team to design a plan that combines email and LinkedIn for better results. Get in touch.

Updated on: 19/09/2025