What's your inbox-to-domain ratio? We're testing 3:1 vs 5:1
ratiorick · 2026-03-19 · 218 views
Been running 3 inboxes per domain for about 6 months and it's been solid — 3.8% reply rate across 12 domains, 36 inboxes total. But I keep hearing people pushing 5 inboxes per domain and I'm curious if anyone has real data on whether that hurts deliverability.
My current setup:
- 12 domains, 3 inboxes each = 36 inboxes
- 15 emails per inbox per day
- Sending through Smartlead
- All warmed for 4+ weeks
- Reply rate: 3.8% average across all inboxes
What I'm considering: Moving to 5 inboxes per domain on 4 of my domains as a test. That would give me 8 extra inboxes and roughly 120 more emails per day without buying new domains. But I'm worried about domain reputation — more inboxes means more total volume from one domain, and Google might not like that.
The math argument for 5:1: Domains cost $10-12/year. Inboxes cost $4-6/month. If I can add 2 inboxes to an existing domain instead of buying a new domain + 3 inboxes, I save the domain cost and reduce DNS setup overhead. But only if deliverability holds.
Has anyone done a proper A/B test on this? Same copy, same list quality, just different inbox-to-domain ratios? I want to see real numbers, not vibes.
Comments (6)
infradan · 2026-03-19
we ran this exact test last quarter. 14 domains, split evenly — 7 at 3:1 and 7 at 5:1. same copy, same audience, same sending platform (Instantly). after 6 weeks the 3:1 domains averaged 4.1% reply rate and the 5:1 domains averaged 3.4%. not a massive gap but it was consistent across all 7 domains in each group. the 5:1 domains also had slightly worse inbox placement on GlockApps — 88% vs 93%.
volumevince · 2026-03-19
I've been running 5:1 for almost a year across 20 domains and haven't noticed any deliverability issues. the key for me is keeping daily volume per inbox LOW — I only do 12 emails per inbox per day. so even at 5 inboxes that's only 60 emails from one domain. I think the total domain volume matters more than the ratio itself. if you're doing 5:1 at 20 emails per inbox that's 100 per domain per day and yeah Google is going to notice that
coldkingdom · 2026-03-19
honestly 3:1 is the sweet spot for most people. I tried going to 4:1 on a few domains and it was fine, but 5:1 started showing cracks after about 4 weeks. the domains didn't get blacklisted or anything dramatic — just a slow decline in inbox placement that compounded over time. went back to 3:1 and things recovered within 2 weeks
agency_pete · PipelineForge · 2026-03-19
We manage 200+ inboxes for clients and we standardize on 3:1. The operational simplicity alone is worth it — easier to track, easier to diagnose when something goes wrong, and clients never push back on domain costs when you explain the deliverability math. We tried 5:1 for two clients and both saw reply rates drop ~15% within a month. Not worth the savings.
ratiorick · 2026-03-19
this is exactly the kind of data I was looking for. sounds like the consensus is 3:1 is safer and the cost savings from 5:1 don't justify the risk. going to stick with 3:1 on my main domains and maybe test 4:1 on two less critical domains to see if there's a middle ground
tina_infra · 2026-03-19
one thing people overlook — at 5:1 you also have 5 inboxes that can potentially get spam complaints, all dragging down the same domain. at 3:1 your blast radius is smaller. had a client whose one bad inbox at 5:1 tanked deliverability for all 4 other inboxes on that domain. with 3:1 you lose less if one inbox goes sideways