What are enterprise email gateways?
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If you're sending B2B email, you've probably noticed that some messages that pass every spam filter still get blocked before they even reach the recipient's inbox. That's often an enterprise email gateway at work.
An enterprise email gateway (also called a Secure Email Gateway, or SEG) is a dedicated security layer that sits in front of a company's internal mail server. Every email sent to that organization passes through the gateway first. If the message looks suspicious, it gets quarantined, stripped, or blocked entirely before any employee ever sees it.
The way it works technically: the company points its MX records at the gateway instead of directly at its mail server. That means all inbound mail routes through the gateway first, gets scanned, then passes along to the real destination. Outbound mail can be routed through the same gateway for policy enforcement and data loss prevention on the way out.
What a gateway actually checks for goes well beyond basic spam filtering. It scans attachments for malware, detects phishing attempts, rewrites or checks links for known threats, and enforces company-wide policies like blocking certain file types or flagging messages that contain sensitive data. Some gateways even detonate suspicious attachments inside an isolated sandbox environment to see what they do before letting the message through.
The major players you'll run into most often in enterprise environments are Proofpoint Email Protection, Mimecast Secure Email Gateway, Barracuda Email Security Gateway, and Cisco Email Security (now part of Cisco's security portfolio). Each has its own reputation scoring, threat intelligence feeds, and filtering logic. That's why the same email can sail through one company's gateway and get blocked at another's.
For senders, this matters a lot. Enterprise gateways are an extra layer of filtering that operates independently from the mailbox provider. You could have perfect authentication, a clean sending reputation, and great content, and still get blocked if a recipient's gateway doesn't like something about your message. Understanding how these systems work is one of the most overlooked parts of B2B deliverability.
If you're regularly emailing corporate addresses and seeing unexplained delivery failures, our SOS hotline is free. Gateway-related blocks have specific patterns, and it's worth a conversation before you start guessing.
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