Tested 8 cold email inbox providers in 90 days. Here is the ranking

provider_tester · 2026-05-02 · 3,120 views

Ran a 90-day structured test across 8 cold email inbox providers. Same campaign copy, same ICP, same sending platform (Instantly). Only variable: inbox infrastructure provider. Here is the ranking by deliverability.

Test setup. 8 providers, 10 inboxes each, 80 inboxes total. Sending 15 cold emails per inbox per day across all 80 inboxes. 90 days of data. Total cold emails sent: 108,000.

Ranking by reply rate (best to worst):

1. PuzzleInbox (pre-warmed): Reply rate 4.3%. Bounce 1.1%. Spam 3.2%. Best overall.

2. Mission Inbox: Reply rate 3.8%. Bounce 1.8%. Spam 8%. Enterprise-tier performance.

3. Mailreef: Reply rate 2.7%. Bounce 2.4%. Spam 11%. Mid-tier.

4. Hypertide (Outlook): Reply rate 2.5%. Bounce 2.3%. Spam 11%. Solid for Microsoft-heavy prospects.

5. Inboxology: Reply rate 2.3%. Bounce 2.8%. Spam 14%. Typical mid-tier.

6. Maildoso: Reply rate 1.9%. Bounce 3.2%. Spam 18%. Shared infrastructure variability.

7. Mailscale: Reply rate 1.7%. Bounce 3.8%. Spam 22%. Budget shared pool risks showing.

8. Cheapinboxes: Reply rate 1.4%. Bounce 4.2%. Spam 25%. Cheapest pricing, worst outcomes.

Cost-per-meeting analysis. Puzzle Inbox pre-warmed: $18 per meeting. Maildoso: $47 per meeting. Cheapinboxes: $68 per meeting. The cheapest inboxes produced the most expensive meetings.

The pre-warming effect. Puzzle Inbox pre-warmed inboxes consistently outperformed self-warmed alternatives by 30-50% in reply rate. The pre-warming is the single biggest deliverability lever.

The shared infrastructure penalty. Providers with shared sending pools (Mailscale, Maildoso, Cheapinboxes) had 2-3x higher spam placement than dedicated infrastructure.

The conclusion. Infrastructure matters. Pay for dedicated pre-warmed inboxes and save on warmup tools, replacement churn, and cost per meeting. The budget savings on cheap providers are illusory.