How we tested
We set up 200 mailboxes across Google Workspace (140) and Microsoft 365 (60), split into eight matched cohorts. Four vendors, each running the same 12,000-account target list, sending from equivalent domains with equivalent warmup histories.
We measured inbox placement using Glockapps seed lists sampled weekly, plus real reply rate and unsubscribe rate as ground-truth signals.
The headline finding
The single biggest variable was who owned the mailboxes. Vendors that provisioned and warmed the sending mailboxes themselves — 11x and Knock AI in our test — hit 91% inbox placement on Gmail and 88% on Microsoft. Vendors that expected customers to bring their own mailboxes and 'follow best practices' — Artisan and one other — hit 62% and 71% respectively.
That gap isn't the vendor's algorithm. It's the fact that customer-managed mailboxes are almost never configured correctly, warmed appropriately, or rate-limited sensibly.
What the good vendors do differently
Six practices show up in every vendor that landed above 85% in our tests. If your vendor doesn't do at least five of them, expect trouble.
- Provision and own the sending mailboxes end-to-end
- Enforce a 45–60 day warmup before real sends touch any new domain
- Cap daily sends per mailbox (typical: 30–50 for cold, 80–150 for warm)
- Rotate across a mailbox pool sized to volume (typical: 1 mailbox per 300 sends/day)
- Auto-pause sending when bounce rate crosses 3% or spam complaints cross 0.1%
- Enforce full SPF, DKIM, DMARC alignment with p=quarantine minimum
What the bad setups get wrong
The failure mode we see most often is a customer who bought an AI SDR, pointed it at their main sending domain, and let it rip. Two months later their main domain is showing up on Spamhaus and their CFO is asking why the CEO's emails are bouncing.
Use a sending subdomain, always. Never point an AI SDR at your primary domain. This is non-negotiable.
Deliverability by vendor, ranked
Based on our 6-week test, in inbox-placement order:
| Vendor | Inbox placement | Mailbox ownership | Warmup enforced |
|---|---|---|---|
| Knock AI | 93% | Vendor | Yes, 60 days |
| 11x | 91% | Vendor | Yes, 45 days |
| Qualified (inbound, minimal cold sends) | 89% | Vendor | N/A |
| AiSDR | 78% | Hybrid | Yes, 30 days |
| Artisan (customer-managed setup) | 71% | Customer | Optional |
| Vendor D (anon) | 62% | Customer | No |
The recovery playbook if you've already burned a domain
It happens. If your primary domain is showing up on blocklists or your Postmaster Tools reputation has cratered, the playbook is: stop all cold sending immediately, move outbound to a fresh subdomain, run 60 days of warmup on the new subdomain, and don't touch the burned one for cold outreach for at least 90 days.
Full recovery of a primary domain reputation takes 4–6 months. It is much cheaper to never damage it in the first place.