Customer.io email service provider (ESP)
According to Live Direct Marketing's daily Tranco top-1M scan (September 11, 2026), Customer.io is the email service provider (ESP) for 454 domains — 0.07% of 631,089 SPF-publishing domains. Detection method: include: mechanisms in the SPF record.
Key facts
- 454 domains in the Tranco top-1M use Customer.io (0.07% of SPF-publishing domains).
- Snapshot date: 2026-09-11 — recomputed every day.
- Strongest TLD: .com (68.1% of its domains).
- Most co-used service: Google Workspace.
- Directory category: email service provider.
Why Customer.io appears in this category
Lifecycle marketing automation for product teams.
We classify it as a email service provider from include: mechanisms in the SPF record matching a maintained provider rule. This is infrastructure attribution from public DNS, not evidence of a direct contract or exclusive use.
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.
Where Customer.io sits in the Tranco ranking
| Tranco rank band | Domains | Share of its base |
|---|---|---|
| Top 1,000 | 0 | 0.0% |
| Top 10,000 | 8 | 1.8% |
| Top 100,000 | 96 | 21.1% |
| Top 1,000,000 | 348 | 76.7% |
| Unranked | 2 | 0.4% |
Top-level domain split
| TLD | Domains | Share | |
|---|---|---|---|
| .com | 309 | 68.1% | |
| .io | 24 | 5.3% | |
| .co | 10 | 2.2% | |
| .casino | 10 | 2.2% | |
| .net | 9 | 2.0% | |
| .org | 7 | 1.5% | |
| .de | 6 | 1.3% | |
| .com.br | 6 | 1.3% | |
| .co.uk | 6 | 1.3% | |
| .no | 5 | 1.1% |
What Customer.io users also use
Cross-tabulated from the same daily snapshot — the other email services seen on the same 454 domains.
| Type | Service | Domains | Share of Customer.io users |
|---|---|---|---|
| Mailbox | Google Workspace | 355 | 78.2% |
| Mailbox | Microsoft 365 | 32 | 7.0% |
| Mailbox | Unknown / Other | 14 | 3.1% |
| Mailbox | Self-Hosted | 11 | 2.4% |
| Mailbox | Generic / unmatched (mail.*) | 10 | 2.2% |
| Mailbox | Generic / unmatched (mx*.*) | 6 | 1.3% |
| ESP | Zendesk | 105 | 23.1% |
| ESP | Mandrill | 76 | 16.7% |
| ESP | SendGrid (Twilio) | 69 | 15.2% |
| ESP | Amazon SES | 67 | 14.8% |
SPF configuration and sender authentication alignment
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) →
Run a free inbox-placement test across Gmail, Outlook, Yahoo and more — see exactly where your mail lands and why.
Frequently asked questions
How does Customer.io affect email deliverability?
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.
How many domains use Customer.io?
454 domains in the Tranco top-1M use Customer.io as their email service provider (ESP) as of September 11, 2026 (0.07% of SPF-publishing domains), according to Live Direct Marketing's daily DNS scan.
Is Customer.io common enough to affect email deliverability testing?
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.
Which countries or TLDs use Customer.io most?
Customer.io's largest TLD is .com (68.1% of its domains); it appears across 50 TLDs in total.