Enterprise SEG Deliverability at Scale: What Production Data Shows

If you sell to mid-market or enterprise companies, a lot of your prospects do not receive email the way a small business does.

Their company runs email through a security filter first — usually something like Proofpoint, Mimecast, Barracuda, or Cisco IronPort. Security teams call these SEGs (secure email gateways). For outbound teams, the practical question is simpler:

Did the company's filter let our email through, or block it before anyone saw it?

That is different from "did the sequencer mark it sent." It is also different from "did it land in the inbox." The filter can accept your message and it still ends up in spam. But if the filter hard-rejects it, nothing downstream matters.

Most cold email tools cannot see that layer. They send from Google or Outlook mailboxes and report bounces when they get them back. They do not run the mail servers, so they do not see accept vs reject at the corporate gateway.

SuperSend does run the send layer — dedicated mail servers and IPs, fully managed. That is how we can measure what happens when outbound mail hits these filters at scale.

This is not a customer case study. No company is named below. The table is real send data from high-volume B2B outbound running on SuperSend infrastructure over the last 30 days.

The Numbers

We grouped sends by which security filter protects the recipient's company, then counted how often mail was hard-rejected (bounced) vs accepted by that filter.

Accepted at the gateway means the company's security filter took the message in. It does not guarantee inbox placement. It does mean the mail was not blocked at the front door.

What stands out

• Mail is not getting blocked at the gateway on every send to these companies.
• Proofpoint and Mimecast acceptance was very high in this window.
• Barracuda was tougher — about 1 in 5 sends hard-rejected — but still not a complete wall.

We also saw replies come back from people at companies protected by all three filters during the same period. A reply is not a deliverability metric, but it confirms mail got far enough for a human to respond.

Why Most Teams Cannot See This

When you email jane@bigcorp.com, the path looks roughly like this:

Your send → BigCorp's security filter → BigCorp's mail server → Jane's inbox

The security filter is the gate. It can:

• Accept the message and pass it along
• Hard-reject it (you get a bounce)
• Defer it and try again later

After acceptance, the message can still land in spam or quarantine. That is a separate problem. But hard rejection is the clearest "you did not get through" signal — and most teams using a sequencer on Gmail mailboxes never see it broken out by filter type.

If a large share of your target accounts sit behind Proofpoint or Mimecast, that blind spot matters. You either assume enterprise domains are unreachable, or you keep sending without knowing where mail is actually failing.

For how this relates to bounce monitoring vs inbox placement testing, those measure different layers. This table is specifically about the gateway step.

Why SuperSend Can Measure It

SuperSend is not just a sequencer. It is outbound infrastructure:

• Dedicated mail servers and IPs, operated by SuperSend
• Sequencing, reply handling, and deliverability monitoring in one place
• Server-level logs: accepted, blocked, deferred, and why

When a corporate filter hard-rejects mail, we see it on the server. When mail is accepted, we know it cleared the gateway. That is a different data layer than clicking send from a Google mailbox.

See email sending infrastructure and deliverability monitoring for how this fits the platform.

What This Shows (and What It Does Not)

What it shows

• Corporate security filters are not automatic blockers for cold outbound at scale.
• Rejection rates differ by filter type — Barracuda was stricter than Proofpoint or Mimecast here.
• When you own the send layer, this is measurable. You are not guessing from a "sent" count.

What it does not show

• Guaranteed inbox placement. Accepted at the gateway ≠ in the primary inbox.
• The same results for every sender. List quality, reputation, volume, and ramp all change outcomes.
• That switching sequencers fixes deliverability without proper sending infrastructure.

These numbers come from one 30-day window on programs running at high volume on SuperSend managed servers. They are useful as a reference point, not a promise for your specific list.

For broader bounce context, see Bounce Rate Benchmarks for Cold Email. For troubleshooting, see How to Diagnose Cold Email Deliverability Problems.

The Bottom Line

1. Enterprise outbound hits security filters. That is normal, not an edge case.
2. Hard rejection at the filter is the clearest failure signal — and most sends in this window cleared it.
3. Not all filters behave the same. Plan for variance, especially with Barracuda-protected domains.
4. You need infrastructure-level visibility to measure any of this. A sequencer on consumer mailboxes cannot.

If you are scaling past mailbox limits and need to see what happens before the inbox, read Dedicated Mail Servers vs Mailboxes for Cold Outbound and More Mailboxes Does Not Fix Cold Email Scale.

Who This Is For

SuperSend is built for teams running high-volume cold outbound as a core channel — not for testing a sequencer on a handful of Google mailboxes for a week.

If your target market includes companies behind Mimecast, Proofpoint, or Barracuda, and you need to know whether mail is getting through the filter — not just whether it left your tool — this is what infrastructure-level sending makes visible.

For a broader enterprise setup checklist, see Enterprise Cold Email Infrastructure Checklist.

Next Step

SuperSend combines dedicated mail servers and IPs, sequencing, bounce categorization, placement testing, domain health, sender health, Super Inbox, and API control in one platform.

Want a quick check before you scale? Use the free email deliverability checker. Want ongoing visibility into acceptance and bounces tied to the send layer? Book a demo or explore SuperSend deliverability.