The SMTP relay
your app can rely on.
A drop-in replacement for your mail server or SMTP provider. Point any app, framework, or server at smtp.lettr.com and get better deliverability, encrypted ports, real-time analytics, and searchable logs — without changing a line of code.
3,000 emails/month free. No credit card required.
What is an
SMTP relay?
An SMTP relay is a mail server that accepts email from your application over the SMTP protocol and forwards it to recipients on your behalf.
Instead of running your own mail server, you point your app at the relay — a host, port, username, and password — and it handles delivery, authentication, queueing, retries, and reputation so your mail reaches the inbox.
- 01 Your app submits a message to the relay over an authenticated SMTP connection.
- 02 Lettr signs it (SPF, DKIM, DMARC), queues it, and manages sending reputation.
- 03 The recipient's mail server accepts it — and every event flows back as analytics.
A relay (or smart host) is sometimes called an SMTP gateway. The job is the same: take outbound mail and deliver it reliably.
Signs you've outgrown
your own mail server.
If any of these sound familiar, a managed relay will save you hours of deliverability work — and a lot of mail in spam folders.
High-volume sending
Your own mail server chokes past a few thousand messages a day. A relay handles queueing, throttling, and retries so volume never becomes an outage.
Inbox placement matters
Reset links, receipts, and alerts have to arrive. A relay brings authentication, reputation monitoring, and feedback loops that a self-hosted server lacks.
Getting off shared IPs
A neighbor's spam shouldn't tank your delivery. Move to a relay with managed pools — and a dedicated IP when you're ready to own your reputation outright.
Apps & servers that only speak SMTP
WordPress, legacy CRMs, ERPs, monitoring tools, and appliances expose only an SMTP screen. A relay is the only way to give them modern deliverability.
One provider for a mixed stack
A WordPress site on SMTP next to a Node service on the API — both routed through Lettr keep analytics, suppression lists, and reputation unified.
Serverless & container sending
When holding an SMTP connection open is awkward, the relay's short-lived auth and our REST API give you a reliable path either way.
Everything a raw mail server isn't.
In one relay.
The same SMTP protocol your code already speaks, backed by the deliverability, security, and visibility a self-hosted server can't give you.
Drop-in SMTP compatibility
A fully standards-compliant relay. Point your existing host, port, and credentials at Lettr and send — no code changes, no rewrites, no new SDK to learn.
Deliverability built in
SPF, DKIM, DMARC, and custom return-paths are first-class. A guided DNS wizard authenticates your domain in minutes so your mail lands in the inbox, not spam.
Encrypted on every port
Implicit TLS on 465 and 2465, STARTTLS on 587 and 2587 — alternate ports for hosts behind restrictive firewalls. API-key authentication, so you can revoke a key anytime without touching a password.
Built to scale
From the first transactional email to millions a month. Automatic queueing and retries absorb spikes, and dedicated IPs are available when you need to own your reputation.
Analytics on every send
Opens, clicks, bounces, and complaints — the visibility a raw SMTP server never gives you. Watch delivery in real time and react to issues before they spread.
Searchable logs
Full-text search across every message the relay has handled. Filter by recipient, status, or subject and find exactly why one email bounced — in seconds.
Sending in
under five minutes.
Three steps. Copy the credentials below into your app or your provider's SMTP settings and send your first email.
Create a free account
Sign up and generate an API key from the dashboard — it doubles as your SMTP password.
Authenticate your domain
The guided DNS wizard walks you through SPF, DKIM, and DMARC in a few minutes — no CNAME guesswork.
Point your app at the relay
Drop in the settings on the right. Any language, framework, or app with an SMTP screen works unchanged.
- Host
- smtp.lettr.com
- Ports
- 465 / 2465 (TLS) · 587 / 2587 (STARTTLS)
- Username
- lettr
- Password
- your API key (lttr_…)
Two ways to send.
One provider.
Lettr speaks both SMTP and a REST API, so you never have to pick a provider by transport. Use whichever fits each part of your stack — the logs, analytics, and suppression list stay unified.
Use the SMTP relay when…
- Existing apps, plugins, or appliances you can’t modify
- Drop-in migration from another SMTP provider
- Tools that only expose host / port / user / pass
- WordPress, CRMs, ERPs, and monitoring systems
Use the REST API when…
- New application code you control
- Unambiguous responses with a message ID
- Idempotency keys to prevent duplicate sends
- Webhooks, templates, and richer error handling
Need the full breakdown? Read SMTP vs. REST API: how to choose.
Need help deciding?
Talk to our team about your email infrastructure, volume needs, or migration from another provider.
Talk to an expertPro
50K emails · $0.80 / 1,000 extra
- Up to 10 sending domains
- 1 inbound route
- Up to 10 webhook endpoints
- No daily sending limit
- Advanced analytics
- 30-day data retention
- No dedicated IPs
Enterprise
Unlimited emails, dedicated IPs, SLA guarantee, SSO/SAML, and a dedicated account manager.
Compare plans
Swipe to compare →| Feature | Free | Pro | Business | Enterprise |
|---|---|---|---|---|
| Monthly emails | 3,000 | 100K emails | 200K emails | Unlimited |
| Daily limit | 100 / day | No limit | No limit | No limit |
| Sending domains | 1 domain | 10 domains | Unlimited | Unlimited |
| Inbound routes | 1 | 10 | Unlimited | |
| Webhook endpoints | 1 | 10 | Unlimited | Unlimited |
| Email API & SMTP | ||||
| Analytics | Basic | Advanced | Advanced | Advanced |
| Data retention | 7 days | 30 days | 30 days | 90 days |
| Team members | ||||
| Dedicated IPs | Add-on | |||
| Priority support | ||||
| SLA guarantee |
SMTP relay questions
Everything developers ask before pointing their app at a relay.
Point your app at smtp.lettr.com.
Get SMTP credentials in minutes and send your first 3,000 emails a month free — no credit card, no code changes.