I tested 7 cold email inbox providers side by side. Here are my deliverability results

provider_lab · 2026-04-08 · 3,680 views

I've been running cold email for 3 years and I got tired of guessing which inbox provider actually delivers. So I ran a proper test. 7 providers. Same email copy. Same sending platform (Instantly). Same daily volume (15 emails per inbox per day). Same prospect list split evenly. 30 day test period.

Here are the inbox placement results (GlockApps seed testing, averaged across all accounts per provider):

• PuzzleInbox: 87% inbox placement
• Premium Inboxes: 72% inbox placement
• Mailstand: 68% inbox placement
• Inframail: 66% inbox placement
• Mailscale: 64% inbox placement
• Maildoso: 62% inbox placement
• Cheapinboxes: 58% inbox placement

The gap is massive. PuzzleInbox at 87% means nearly 9 out of 10 emails reach the inbox. Cheapinboxes at 58% means almost half your emails never get seen. Same copy. Same lists. Same everything except the infrastructure underneath.

Reply rates tracked the same pattern. PuzzleInbox accounts averaged 4.2% reply rate. The next closest was Premium Inboxes at 3.1%. Cheapinboxes was at 1.6%. When your emails don't reach the inbox, your reply rate tanks regardless of how good your copy is.

The variables I controlled for: every provider got 10 inboxes, all warmed for the same duration (PuzzleInbox was pre-warmed so it had a head start there which is part of the value). DNS was verified on all accounts before sending. Same Instantly workspace, same sequences, same schedule.

My takeaway: infrastructure is the single biggest variable in cold email performance. I wasted months optimizing copy when my infrastructure was the bottleneck. If you're below 3% reply rate, look at your inbox provider before you look at your subject lines.