According to Live Direct Marketing's daily Tranco top-1M scan (July 13, 2026), Customer.io is the email service provider (ESP) for 458 domains — 0.07% of 617,702 SPF-publishing domains. Detection method: include: mechanisms in the SPF record.
Customer.io is a customer data platform and email marketing automation service that sends transactional and campaign emails on behalf of its users. It operates as a standalone commercial SaaS platform.
Customer.io's detection in email infrastructure reveals a particular pattern: the platform is more prevalent in the long tail of smaller and mid-market organisations than in globally dominant brands, is predominantly used in the .com namespace, and frequently appears alongside Google Workspace deployments. For deliverability professionals, this means filtering and reputation decisions must account for the heterogeneous sender base using Customer.io — reputation at the platform's IP addresses reflects the combined sending practices of many small organisations rather than a single high-volume brand.
| Tranco rank band | Domains | Share of its base |
|---|---|---|
| Top 1,000 | 0 | 0.0% |
| Top 10,000 | 9 | 2.0% |
| Top 100,000 | 104 | 22.7% |
| Top 1,000,000 | 331 | 72.3% |
| Unranked | 14 | 3.1% |
| TLD | Domains | Share | |
|---|---|---|---|
| .com | 309 | 67.5% | |
| .io | 25 | 5.5% | |
| .casino | 10 | 2.2% | |
| .co | 10 | 2.2% | |
| .net | 9 | 2.0% | |
| .org | 8 | 1.7% | |
| .de | 6 | 1.3% | |
| .co.uk | 6 | 1.3% | |
| .com.br | 6 | 1.3% | |
| .nl | 5 | 1.1% |
Cross-tabulated from the same daily snapshot — the other email services seen on the same 458 domains.
| Type | Service | Domains | Share of Customer.io users |
|---|---|---|---|
| Mailbox | Google Workspace | 355 | 77.5% |
| Mailbox | Microsoft 365 | 35 | 7.6% |
| Mailbox | Unknown / Other | 13 | 2.8% |
| Mailbox | Self-Hosted | 12 | 2.6% |
| Mailbox | Generic / unmatched (mail.*) | 10 | 2.2% |
| Mailbox | Generic / unmatched (mx*.*) | 6 | 1.3% |
| ESP | Zendesk | 105 | 22.9% |
| ESP | Mandrill | 74 | 16.2% |
| ESP | SendGrid (Twilio) | 71 | 15.5% |
| ESP | Mailgun | 67 | 14.6% |
Organisations using Customer.io to send email typically add the platform's SPF include mechanism to their SPF records to authorise its sending infrastructure. This pattern is directly observable in email authentication records across the web and defines Customer.io's footprint in email data. Correct SPF, DKIM, and DMARC alignment is essential for any organisation using Customer.io to maintain sender reputation and avoid filtering issues.
See the full Customer.io data page (KPI cards, trend, sample domains) →
Customer.io is an email marketing automation platform used by many organisations; filtering decisions depend on proper authentication and the individual sender's compliance practices, since IP reputation reflects the combined behaviour of all Customer.io users.
458 domains in the Tranco top-1M use Customer.io as their email service provider (ESP) as of July 13, 2026 (0.07% of SPF-publishing domains), according to Live Direct Marketing's daily DNS scan.
Customer.io accounts for 0.07% of SPF-publishing domains in the top 1M. Whether you should seed a test mailbox there depends on how much of your audience it represents — the co-usage table above shows the services it appears alongside most.
Customer.io's largest TLD is .com (67.5% of its domains); it appears across 49 TLDs in total.