AuthSMTP email service provider (ESP)
According to Live Direct Marketing's daily Tranco top-1M scan (September 16, 2026), AuthSMTP is the email service provider (ESP) for 959 domains — 0.15% of 634,905 SPF-publishing domains. Detection method: include: mechanisms in the SPF record.
Key facts
- 959 domains in the Tranco top-1M use AuthSMTP (0.15% of SPF-publishing domains).
- Snapshot date: 2026-09-16 — recomputed every day.
- Strongest TLD: .com (55.1% of its domains).
- Most co-used service: Microsoft 365.
- Directory category: email service provider.
Why AuthSMTP appears in this category
UK SMTP relay for SMB outbound.
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.
AuthSMTP is an email service provider that senders authorise through SPF include mechanisms. It facilitates outbound mail for customers across various sectors, with its usage declared in sending domain SPF records.
AuthSMTP's footprint sits predominantly in the long tail of medium-to-small websites rather than among the largest global brands, with strong concentration in the .com namespace. It frequently appears alongside Microsoft 365 configurations, suggesting adoption among SMBs and enterprises running hybrid mail environments. For senders and recipients alike, this means AuthSMTP-authorised mail originates from a diverse, distributed set of smaller domains rather than from a small number of heavily-trafficked senders.
Where AuthSMTP sits in the Tranco ranking
| Tranco rank band | Domains | Share of its base |
|---|---|---|
| Top 1,000 | 1 | 0.1% |
| Top 10,000 | 19 | 2.0% |
| Top 100,000 | 136 | 14.2% |
| Top 1,000,000 | 793 | 82.7% |
| Unranked | 10 | 1.0% |
Top-level domain split
| TLD | Domains | Share | |
|---|---|---|---|
| .com | 528 | 55.1% | |
| .co.uk | 80 | 8.3% | |
| .org | 65 | 6.8% | |
| .nl | 58 | 6.0% | |
| .es | 24 | 2.5% | |
| .net | 23 | 2.4% | |
| .org.uk | 19 | 2.0% | |
| .com.au | 18 | 1.9% | |
| .ca | 12 | 1.3% | |
| .ie | 11 | 1.1% |
What AuthSMTP users also use
Cross-tabulated from the same daily snapshot — the other email services seen on the same 959 domains.
| Type | Service | Domains | Share of AuthSMTP users |
|---|---|---|---|
| Mailbox | Microsoft 365 | 403 | 42.0% |
| Mailbox | Google Workspace | 202 | 21.1% |
| Mailbox | Self-Hosted | 65 | 6.8% |
| Mailbox | Mimecast | 59 | 6.2% |
| Mailbox | Unknown / Other | 52 | 5.4% |
| Mailbox | Proofpoint | 24 | 2.5% |
| ESP | Amazon SES | 116 | 12.1% |
| ESP | SendGrid (Twilio) | 112 | 11.7% |
| ESP | Mailchimp | 98 | 10.2% |
| ESP | Zendesk | 95 | 9.9% |
SPF authorisation and long-tail reputation dynamics
AuthSMTP's concentrated presence on smaller domains means senders authorising it do not inherit reputation from large, high-volume senders. This distributes AuthSMTP's reputation signal across many smaller sources rather than concentrating it; consequently, senders should prioritise domain-level authentication (DKIM and DMARC) and list hygiene as more reliable deliverability levers than SPF alone.
See the full AuthSMTP data page (KPI cards, trend, sample domains) →
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Frequently asked questions
Do I need to test against AuthSMTP for email deliverability?
If you are not using AuthSMTP to send mail, its presence in SPF records has no direct impact on your deliverability. If you do use it, prioritise DKIM and DMARC authentication and sender reputation rather than relying on SPF alone.
How many domains use AuthSMTP?
959 domains in the Tranco top-1M use AuthSMTP as their email service provider (ESP) as of September 16, 2026 (0.15% of SPF-publishing domains), according to Live Direct Marketing's daily DNS scan.
Is AuthSMTP common enough to affect email deliverability testing?
AuthSMTP accounts for 0.15% 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 AuthSMTP most?
AuthSMTP's largest TLD is .com (55.1% of its domains); it appears across 58 TLDs in total.