1.5x More Leads: How C17 Lab Broke Its Inbox Burnout Cycle on InboxKit

Before InboxKit, C17 Lab ran outbound infrastructure across a mix of providers for its client base. The pattern repeated everywhere: mailboxes went bad quickly, forcing the team to refresh infrastructure on a constant cycle. Every refresh meant more warm-up time, and more warm-up time meant delayed campaign launches for clients who were already waiting on results.

Before InboxKit, we were using different providers at C17. We used to notice our mailboxes go bad really quickly, they burn up pretty fast, and we had to freshen up our infrastructure regularly for clients, which increased warm-up duration and delays in client campaigns, making the clients unhappy.

— The team at C17 Lab

The onboarding was fast enough that it surprised C17 Lab's own team. Understanding the platform took one day. Placing the first orders took the same day.

My team just needed one day to understand how InboxKit works, how the platform works. From day one, we were able to place orders for inboxes. What surprised me was how quick the platform is. You place an order, set up all the configuration, and within hours you see inboxes coming in.

— The team at C17 Lab

That speed carried into daily operations. Running 12,000+ mailboxes doesn't mean logging in every day to manage them.

The way InboxKit has set up the back end is really good. My team doesn't log in day to day. They log in once a week, see how things are showing up, and keep it as is. If they want to place more orders, they log in, place the order, and within a couple of hours you see inboxes coming in.

— The team at C17 Lab

One incident stood out as proof the switch mattered. A top client, one that regularly hit 3x their target KPIs, saw their infrastructure fail completely on another vendor. Leads dropped to zero. Pushing send volume to 100 to 200 emails a day didn't move the needle.

C17 Lab moved that account onto InboxKit. Results came back the day the campaign relaunched.

There was this one instance where one of our clients who used to get massive results, like 3x the KPIs they wanted, suddenly their infra went down completely and we were getting zero leads. We tried increasing the volume, still nothing. So we switched to InboxKit. On day one of launching the campaign, we were back on track again, getting 3x the leads we were getting before. The client was happy, the CSM was happy, our growth managers and GTMs were happy. That really made us look good.

— The team at C17 Lab

Across the account, C17 Lab now runs 12,645 mailboxes on InboxKit (11,648 active) across 5,510 domains, roughly 2 to 3 inboxes per domain, in line with how the team described their own setup.

On average, it's around 1.5 times more leads we are getting with InboxKit than we were getting with other providers. For some clients, it's even 2x, so it depends.

— The team at C17 Lab

Inbox shelf life turned out to matter as much as lead volume. Mailboxes that last longer mean fewer refresh cycles, and fewer refresh cycles mean faster launches and better margins.

The refreshment cycle of the infra we used to have earlier has increased with InboxKit, because the shelf life of the inboxes is much higher. That has really impacted our profitability and our speed to launching campaigns, because now we don't need to wait for refreshment cycles again and again.

— The team at C17 Lab

• Client campaigns stop stalling. Mailbox burnout used to mean warm-up delays on every refresh cycle. Longer-lasting inboxes mean fewer refreshes and fewer excuses clients have to walk.
• Onboarding takes a day, not a quarter. C17 Lab's team learned the platform and placed its first orders on day one, with inboxes provisioning within hours of each order.
• Automation is what makes 12,000+ mailboxes manageable. New inboxes route automatically into C17 Lab's sending tool. That's the one feature the team said they could not lose.
• Recovery is now measured in hours, not weeks. When one client's infrastructure failed on another vendor, moving that account to InboxKit restored full performance on day one of the relaunch.

For a team managing infrastructure across dozens of clients at once, the feature C17 Lab relies on most is automation, specifically, automatically exporting new inboxes into their sending tool.

The automations, if those disappeared, like exporting inboxes to Smartlead automatically, I would cry, and so would my team. That would be the feature that would hurt us the most if it disappeared tomorrow.

— The team at C17 Lab

For the operational side of running this across many clients at once, our agency workflows guide covers how agencies structure multi-client infrastructure day to day.

C17 Lab is a B2B outbound and revenue engineering agency founded by Enzo Carasso. The team builds outbound systems for B2B companies, books meetings with its own SDRs, and then installs the system in-house, and reports having generated $42M+ in revenue for clients as of August 2026 while sending 2 to 3 million emails and generating 3,000+ leads a month across its book of business.

If infra is the real bottleneck standing between you and the results you're looking for, just try InboxKit. If you're worried it won't work, buy a small batch of inboxes, send at least 3,000 emails, and see. I'm confident you'll see an increase in results, an increase in replies. You need to pay for inboxes to run outbound campaigns anyway, so you might as well try a new provider. My team loves working with them.

— The team at C17 Lab, rating InboxKit 9/10

Running outbound for clients?

InboxKit provides the mailboxes, domains, automated DNS, warmup, and white label options that let agencies fix deliverability fast and keep it stable at scale. See plans and pricing or explore our agency email solution.