Unknown / Other mailbox provider
According to Live Direct Marketing's daily Tranco top-1M scan (September 18, 2026), Unknown / Other is the mailbox provider for 45,049 domains — 6.67% of 675,783 domains with MX records. Detection method: primary MX record (lowest preference).
Key facts
- 45,049 domains in the Tranco top-1M use Unknown / Other (6.67% of domains with MX records).
- Snapshot date: 2026-09-18 — recomputed every day.
- 30-day share change: -0.12 pp.
- Strongest TLD: .com (30.5% of its domains).
- Region skew: Global / US-skewed.
- Most co-used service: Amazon SES.
- Directory category: generic unresolved infrastructure.
Why Unknown / Other appears in this category
MX targets that did not match any known pattern and don't follow the generic mail./mx./smtp. naming convention.
We classify it as a generic unresolved infrastructure from primary MX record (lowest preference) matching a maintained provider rule. This is infrastructure attribution from public DNS, not evidence of a direct contract or exclusive use.
Unknown / Other is the category assigned to mailbox providers that do not match known major services like Gmail, Microsoft 365, Proton Mail, or Apple Mail. This includes smaller email hosting services, custom mail server implementations, and organisations running their own infrastructure. The label reflects the diversity of smaller providers across global networks rather than a single entity or operator.
For senders, this category matters because it represents a fragmented landscape of receiving systems with varying levels of email configuration maturity. Recipients using these services may experience inconsistent filtering, authentication enforcement, and inbox placement depending on how well their particular provider implements industry standards. Marketers and systems administrators need to recognise that email destined for this broad set of providers faces a wider range of potential delivery challenges than mail to consolidated providers.
Where Unknown / Other sits in the Tranco ranking
| Tranco rank band | Domains | Share of its base |
|---|---|---|
| Top 1,000 | 45 | 0.1% |
| Top 10,000 | 399 | 0.9% |
| Top 100,000 | 4,279 | 9.5% |
| Top 1,000,000 | 39,317 | 87.3% |
| Unranked | 1,009 | 2.2% |
Top-level domain split
| TLD | Domains | Share | |
|---|---|---|---|
| .com | 13,750 | 30.5% | |
| .de | 3,460 | 7.7% | |
| .net | 2,579 | 5.7% | |
| .org | 1,702 | 3.8% | |
| .ru | 1,616 | 3.6% | |
| .pl | 1,433 | 3.2% | |
| .jp | 1,256 | 2.8% | |
| .it | 998 | 2.2% | |
| .fr | 919 | 2.0% | |
| .co.uk | 857 | 1.9% |
What Unknown / Other users also use
Cross-tabulated from the same daily snapshot — the other email services seen on the same 45,049 domains.
| Type | Service | Domains | Share of Unknown / Other users |
|---|---|---|---|
| ESP | Amazon SES | 1,547 | 3.4% |
| ESP | SendGrid (Twilio) | 1,004 | 2.2% |
| ESP | Mailchimp | 948 | 2.1% |
| ESP | Mailjet (Sinch) | 925 | 2.1% |
| ESP | Mailgun | 840 | 1.9% |
| ESP | Zendesk | 826 | 1.8% |
| ESP | Mandrill | 816 | 1.8% |
| ESP | Trend Micro Email Security | 591 | 1.3% |
| ESP | Salesforce | 590 | 1.3% |
| ESP | Brevo (ex-Sendinblue) | 494 | 1.1% |
Configuration diversity and authentication enforcement
Since Unknown / Other comprises many independent or smaller-scale implementations, these mail servers show significant variation in how strictly they enforce authentication standards like SPF, DKIM, and DMARC. Some smaller providers may lack proper SPF configuration entirely, whilst others apply strict validation; this inconsistency means that sender reputation checks and alignment signals receive variable weight. When Unknown / Other is frequently paired with Amazon SES in data, it often reflects SMEs and startups using SES infrastructure but routing mail through their own domains — a configuration that succeeds only when the sender maintains rigorous authentication setup across all three protocols.
See the full Unknown / Other data page (KPI cards, trend, sample domains) →
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Frequently asked questions
What is Unknown / Other in email deliverability data?
Unknown / Other is a category for mailbox providers not identified as major services—smaller hosting platforms, custom implementations, and independent infrastructure. This category reflects the long tail of email infrastructure across diverse, fragmented receiving systems with varying levels of authentication and filtering enforcement.
How many domains use Unknown / Other?
45,049 domains in the Tranco top-1M use Unknown / Other as their mailbox provider as of September 18, 2026 (6.67% of domains with MX records), according to Live Direct Marketing's daily DNS scan.
Is Unknown / Other common enough to affect email deliverability testing?
Unknown / Other accounts for 6.67% of domains with MX records 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.
Is Unknown / Other growing or declining?
Unknown / Other is declining — its share moved -0.12 pp over the last 30 days (as of September 18, 2026).
Which countries or TLDs use Unknown / Other most?
Unknown / Other's largest TLD is .com (30.5% of its domains); it appears across 501 TLDs in total.