SendPost SMTP Settings and Configuration (2026)
Native SMTP relay with documented endpoints
SendPost provides a full SMTP relay service at smtp.sendpost.io on ports 25, 587, and 2525 with STARTTLS encryption, publicly documented at docs.sendpost.io. Access requires either a Free Proof of Concept (trial mode) or a paid production commitment following a sales demo. Unlike self-serve providers, there is no instant signup; unlike pure enterprise sales, endpoint values are transparent.
SendPost is an email deliverability platform and cloud MTA from the same parent company as SendX, positioned specifically for ESPs, high-volume senders, and deliverability specialists. It powers 300M+ emails per month for high-volume customers and pitches itself against SendGrid, Mailgun, and SparkPost on visibility and MTA-level control. Where SendX is a marketing-first campaign platform, SendPost is the transactional infrastructure sibling with a real SMTP relay.
Quick reference
| SMTP hostname | smtp.sendpost.io |
|---|---|
| Ports supported | 25, 587 (recommended), 2525 (fallback) |
| Encryption | STARTTLS (RFC 3207) on all ports |
| Auth | SMTP username + password (issued from developer portal) |
| API auth (parallel) | X-SubAccount-ApiKey header (sub-account) or X-Account-APIKey (account-level) |
| Delivery methods | SMTP relay + REST Email API |
| SDKs | 14+ programming languages officially supported |
| Access model | Free Proof of Concept, then demo + upfront commitment for production |
| Dedicated IPs | Included in production plans |
| Scale profile | Built for 300M+ emails/month workloads |
| Sandbox mode | Verification required for from-addresses on new accounts |
SMTP configuration
SendPost SMTP relay
For transactional and marketing email at scale with MTA-level control and dedicated IP
Setup steps
- Start a Free Proof of Concept at
sendx.io/sendpost. SendPost’s PoC lets you connect it alongside your current sending infrastructure to see routing insights and AI-powered recommendations before committing. - Complete the sales onboarding: SendPost’s team will run a demo, discuss your use case, and provision your account. Most teams are live within a day.
- Register your SMTP credentials at the SendPost developer portal (linked from
docs.sendpost.io). SendPost issues an SMTP username and password, scoped to your sub-account, alongside API keys for the parallel Email API. - Verify your sending domain(s) via the developer portal. New accounts start in sandbox mode where only verified from-addresses can send.
- Configure your application with
smtp.sendpost.io, port 587 (or 25 / 2525 as fallbacks), your issued SMTP username, and password. STARTTLS is negotiated automatically on all three ports. - Set up SPF and DKIM DNS records per the values shown for each verified sender domain. SendPost enforces DKIM alignment for full deliverability signals.
- Send a test message. Check the developer portal analytics for opens, clicks, bounces, complaints, and MTA-level routing traces (SendPost’s differentiator versus commodity relays).
Access model: PoC first, commitment for production
SendPost’s access flow is unusual for the transactional space and worth understanding upfront :
- Free Proof of Concept. SendPost markets a no-commitment PoC where they connect their infrastructure alongside your existing setup, mirror your sends, and show you delivery data, routing insights, and AI-powered recommendations. No code changes required for the PoC itself.
- Production commitment. Moving from PoC to production requires a formal sales conversation and an upfront financial commitment. There is no self-serve signup with a credit card as with Postmark or SES.
- Sub-account architecture. Production accounts include a sub-account model (X-SubAccount-ApiKey vs X-Account-APIKey) designed for teams operating multiple sending brands, tenants, or business units under one master account.
This model suits ESPs, agencies operating multiple client brands, and high-volume senders who need MTA-level visibility. It is a mismatch for small teams needing SMTP running by end of day.
Deliverability model
SendPost is positioned specifically on deliverability rather than the send-and-forget model of commodity SMTP relays. Notable features :
- Dedicated IPs included in production plans, with automated IP warmup for new IPs.
- Real-time ISP signals and reputation tracking per IP and per sending domain.
- AI-driven deliverability alerts flagging reputation drift, bounce rate spikes, and blocklist entries as they happen.
- MTA-level controls for routing decisions (choose which IPs handle which traffic streams, throttle per-ISP, etc.).
- Webhooks for opens, clicks, bounces, and complaints with per-event enrichment.
- Central issue tracking to connect the dots across delivery failures and prevent recurrence.
Common setup gotchas
No self-serve signup with a credit card
SendPost cannot be signed up for and sending within an hour like Postmark, SES, or MailerSend. The Free PoC is fastest, but even that requires SendPost’s team to configure the mirror setup on their end. If speed of activation is the primary constraint, choose a self-serve provider instead and consider SendPost later when volume justifies the commitment.
Sandbox mode blocks unverified sender addresses
New SendPost accounts start in sandbox mode, sends are restricted to verified from-addresses only. Before production launch, verify every domain and sender you plan to send from. Test emails to arbitrary addresses will fail with clear errors during sandbox mode, which is by design (prevents accidental production sends from a not-yet-warmed setup).
Sub-account API key model is required for multi-tenant setups
SendPost’s authentication uses two distinct API keys: X-SubAccount-ApiKey for per-sub-account operations (sending, domain verification) and X-Account-APIKey for account-level operations (creating sub-accounts, billing). Teams operating multiple client brands under one SendPost account need to design their integration around this split, using the wrong key type returns authorization errors that can be confusing without context.
Overlap with SendX naming causes confusion
SendPost and SendX come from the same parent company but are distinct products with different pricing, access flows, and endpoints. SendX is marketing-first (campaigns, newsletters, drip); SendPost is transactional infrastructure (SMTP relay, MTA controls). Do not assume features of one apply to the other, and specifically, do not attempt to use SendX SMTP for anything (it does not exist).
Commitment pricing means no public rate card
SendPost does not publish per-1,000 pricing. Effective cost per email is negotiated during the sales cycle based on your projected volume and included features (dedicated IPs, sub-account count, support tier). Ask for the effective per-1,000 rate during your PoC review to compare against benchmark providers like Amazon SES ($0.10 per 1,000) or Postmark ($1.25 per 1,000).
If SendPost’s commitment model does not fit
Self-serve SMTP alternatives with public pricing and instant signup :
- Postmark-transactional-only, simple SMTP, $1.25 per 1,000. Best for boutique to mid-volume.
- Amazon SES-cheapest at scale ($0.10 per 1,000), self-serve, AWS-native.
- MailerSend-transactional focus with a 500/mo free tier.
- SendGrid-larger scale with dedicated IPs available, richer marketing overlap.
- Mailgun-developer-focused, per-email pricing, US and EU regions.
Frequently asked questions
What is the SendPost SMTP hostname and port?
The SendPost SMTP hostname is smtp.sendpost.io, publicly documented at docs.sendpost.io. Supported ports: 25 (unencrypted or upgraded via STARTTLS), 587 (STARTTLS, recommended), and 2525 (fallback when standard ports are blocked). STARTTLS is negotiated per RFC 3207 on all three ports.
Can I sign up for SendPost with a credit card?
Not for production. SendPost offers a Free Proof of Concept where they connect their infrastructure alongside your current setup to demonstrate value, no commitment required for the PoC itself. Moving to production requires a formal sales demo and an upfront financial commitment. If self-serve credit-card signup is a hard requirement, use Postmark, Amazon SES, or MailerSend instead.
How is SendPost different from SendX?
Both come from the same parent company but target different use cases. SendX is a marketing platform for newsletters, campaigns, drip automation, and landing pages, targeting small-to-mid businesses. SendPost is an email deliverability platform and cloud MTA targeting ESPs and high-volume senders, with SMTP relay, MTA-level controls, and AI-driven deliverability insights. SendX has no SMTP relay; SendPost does.
Does SendPost include dedicated IPs?
Yes. Dedicated IPs are included in production plans along with automated IP warmup. This is a differentiator versus self-serve providers where dedicated IPs are typically a paid add-on ($30-80/month extra). The trade-off is that production access itself requires the sales-negotiated commitment.
What is the difference between X-SubAccount-ApiKey and X-Account-APIKey?
SendPost uses two authentication scopes. X-SubAccount-ApiKey authenticates sub-account-level operations: sending email, verifying sending domains, and configuring per-brand settings. X-Account-APIKey authenticates account-level operations: creating and managing sub-accounts, billing changes, and cross-tenant reporting. Teams operating multiple client brands under one SendPost account use both keys depending on operation type. Using the wrong scope returns authorization errors.
Do I have to use the API or can I stick with SMTP?
Either works. SendPost’s SMTP relay at smtp.sendpost.io supports standard SMTP clients with no code changes, just point your existing mail library at the endpoint with your issued credentials. The REST Email API provides more granular control for structured metadata, template management, and event-triggered sends. Both hit the same delivery infrastructure and share deliverability signals.
What SDKs does SendPost officially support?
SendPost publishes SDKs in 14+ programming languages covering the common transactional stack: Node.js, Python, PHP, Ruby, Java, Go, .NET, and others. The SDKs wrap the REST Email API rather than SMTP (SMTP works with any standard mail library), and provide typed request/response models, retry logic, and webhook verification helpers.
This review follows our email infrastructure testing methodology. We disclose affiliate relationships in our editorial independence policy.
