What is a cloud filtering service (Proofpoint, Mimecast, Barracuda)?

Your email hits your recipient's mail server. Except it doesn't. First it goes to a security checkpoint. That checkpoint is a cloud filtering service.

Here's how it works. Your recipient's IT team points their MX record to a service like Proofpoint, Mimecast, or Barracuda instead of their own mail server. Your email lands at Proofpoint (or whichever service they use). Proofpoint scans it, filters threats, and if it passes, forwards the clean message to the recipient's actual mail server. If it looks malicious, Proofpoint stops it before it ever reaches the company.

What these services actually scan for:

Malware signatures (known bad files). Heuristics (behavior that looks suspicious even if it's not a known virus). Machine learning models trained on millions of emails. URLs rewritten to click through the filter first (so malicious links get caught if you click). Sandboxing that detonates suspicious attachments in a safe container to see if they're truly dangerous. Threat intelligence feeds from thousands of companies sharing attack data globally.

What's different between the three:

Proofpoint is known for advanced threat protection and targeted attack defense. It's the heavyweight option. Mimecast offers email security plus archiving and disaster recovery (it's not just filtering, it's preservation). Barracuda is the cost-conscious choice, popular with mid-market organizations that need solid protection without the enterprise price tag.

What this means for your sending reputation:

Your message has to pass through their filter rules, not just the recipient's. The filter service has its own scoring logic, block lists, and policy settings. If you're on a reputation blocklist, Proofpoint might block you before the message even reaches the company's mailbox. Authentication requirements (SPF, DKIM, DMARC) still matter, but now they matter at two layers instead of one. Some filters rewrite your links or check your attachments, which can affect how your email renders.

Inbound and outbound filtering:

But most companies use the cloud filter for inbound mail (to protect themselves from threats coming in). Some also route outbound mail through it (so the company's own email gets scanned before it leaves). If you're sending to someone whose outbound filter is active, your reply might be delayed or blocked by their own security scanning. This matters most for security-sensitive industries.

Your next step: if you're having delivery trouble with certain organizations, check whether they use a cloud filter and what their IP reputation requirements are. If you can, ask them directly (especially with prospect research) whether Proofpoint, Mimecast, or Barracuda is in their mail flow. That'll help you diagnose whether it's a filter policy or your sender reputation.