Open source email API comparison
Three email platforms you can read and run yourself: Retransmit, useSend, and Plunk. This is how they differ on license, pricing, features, and what self-hosting actually takes.
By the Retransmit team. Last updated .
Retransmit builds one of the products compared here. Every competitor number was checked on the vendor's own site on the date above, and nothing on this page is paid placement.
Proprietary email APIs ask you to trust a black box with your sender reputation and your users' addresses. The open source options remove that trade. All three tools here expose a transactional API, verify domains with standard DNS records, and can be self-hosted with Docker. They differ in scope. Retransmit is a focused transactional pipeline. useSend adds a marketing suite. Plunk adds automations and inbound. All prices below were checked on September 1, 2026.
Why open source for email
Email infrastructure is sticky. Your domain reputation, suppression lists, and delivery history live with the provider, and moving is painful. Open source changes the exit cost: if the cloud product changes pricing or shuts down, you deploy the same code yourself and keep sending. It also means you can audit exactly what happens to recipient data, which shortens compliance conversations.
License and repository
License matters if you plan to offer email sending to your own customers. AGPL-3.0 requires publishing your modifications when you serve the software over a network. Check with counsel if you are embedding.
| Retransmit | useSend | Plunk | |
|---|---|---|---|
| Repository | github.com/jpainam/retransmit | github.com/usesend/useSend | github.com/useplunk/plunk |
| License | AGPL-3.0, with an MIT licensed SDK | AGPL-3.0 | AGPL-3.0 |
| What is in the repo | API, dashboard, queue, and SDK | Full platform | API, workers, and dashboard |
| Underlying sender | Amazon SES | Amazon SES | Not disclosed |
Cloud pricing
All three fund development with a hosted cloud. The models differ: useSend and Plunk meter monthly, Retransmit is prepaid.
| Retransmit | useSend | Plunk | |
|---|---|---|---|
| Free tier | Self-hosted, every feature | 3,000 emails a month, 1 domain, 1 seat | 1,000 emails a month, branded |
| Paid | Prepaid credits, pay per send | $0.40 per 1,000 transactional, $10 a month minimum | $1.00 per 1,000, all features |
| Contacts | Not stored | Unlimited on paid | Unlimited, free |
| Payment | Bank transfer, mobile money, local currency | Card | Card |
Sources: useSend, Plunk pricing
Feature comparison
Scope is the real differentiator. Match the tool to whether you need transactional only, transactional plus marketing, or lifecycle automation.
| Feature | Retransmit | useSend | Plunk |
|---|---|---|---|
| Transactional API | Yes | Yes | Yes |
| Batch sending | Yes, 10,000 per request | Yes | Not documented |
| Scheduling | Not yet | Yes | Campaigns only |
| Marketing campaigns | Not yet | Yes | Yes |
| Automations | Not yet | No | Yes |
| Inbound email | Not yet | No | Yes |
| Contacts | Not yet | Yes | Yes |
| SMTP relay | Not yet | Yes | Yes |
| Webhooks | 9 events, HMAC signed, 8 retries | Yes | Yes |
| SDKs | Node.js, plus REST | TypeScript, Python, Go, PHP | Node.js |
What self-hosting takes
All three need a Postgres database, a place to run containers, and Amazon SES credentials for the actual sending, except Plunk, which does not disclose its upstream. Plan for DNS records per sending domain, SPF and DKIM at minimum, and SES production access approval from AWS, which usually takes a day.
Retransmit is the smallest deployment of the three because it is the smallest product. The API, dashboard, queue worker, and SDK live in one repository, and the hosted cloud and a self-hosted instance are interchangeable behind a base URL.
Which one should you pick
- Pick Retransmit for transactional sending at volume: batches, queues, logs, and signed webhooks, with prepaid billing in the cloud.
- Pick useSend to replace both a transactional API and a newsletter tool with one open source app.
- Pick Plunk for lifecycle email: automations, segments, and inbound replies feeding one contact list.
Read the code, then send with it. Self-host for free, or start on the cloud with prepaid credits.
Related resources
Retransmit vs useSend
The two SES-based open source stacks in detail.
Retransmit vs Plunk
Transactional focus against an all-in-one marketing platform.
Best Resend alternatives in 2026
The wider roundup, including proprietary options.
Retransmit quickstart
Verify a domain, grab an API key, and send your first email in under five minutes.
Questions people ask
What is the best open source alternative to Resend or SendGrid?
It depends on scope. Retransmit for a focused transactional API with batching and signed webhooks. useSend for transactional plus marketing campaigns and contacts. Plunk for automations, segments, and inbound email. All three self-host with Docker.
Are open source email APIs really free?
The software is. You still pay for servers, a Postgres database, and the underlying sending, typically Amazon SES at $0.10 per 1,000 emails. For most teams that lands far below managed pricing at volume.
Does AGPL affect my product if I self-host one of these?
AGPL-3.0 requires you to publish your modifications if you offer the software to others over a network. That applies to all three platforms here, Retransmit included. Internal use for your own sending is generally fine, but embedding one in a customer-facing product needs legal review.
Do I need my own Amazon SES account to self-host?
For Retransmit and useSend, yes: you bring SES credentials and request production access from AWS. That also means the sending quota and reputation belong to your AWS account, not to a vendor.