Quick comparison table
| Feature | RelayPost | SendGrid | Postmark | Resend | Mailgun |
|---|---|---|---|---|---|
| REST API | ✓ | ✓ | ✓ | ✓ | ✓ |
| SMTP relay | ✓ | ✓ | ✓ | ✓ | ✓ |
| Transactional email | ✓ | ✓ | ✓ | ✓ | ✓ |
| Campaign email | ✓ | ✓ (add-on) | ✓ (Bulk API) | ✓ (Broadcasts) | ✗ |
| DKIM/SPF auto-setup | ✓ | Manual | ✓ | ✓ | Manual |
| Webhooks | ✓ | ✓ | ✓ | ✓ | ✓ |
| Email validation | ✗ | ✓ (paid) | ✗ | ✗ | ✓ (paid) |
| Contact management | ✓ | ✓ | ✗ | Limited | ✗ |
| Unlimited team members | ✓ | ✗ | ✗ | ✗ | ✗ |
| Dedicated IP | Pro plan | $89/mo | $50/mo (Pro+, 300K+) | $30/mo (Scale+) | Paid add-on |
Pricing comparison (100K emails/month)
| Provider | Free tier | 10K/mo | 100K/mo | Pricing model |
|---|---|---|---|---|
| RelayPost | 1,000/mo | $5/mo | $25/mo | Monthly plans |
| SendGrid | 100/day | $19.95/mo | $89.95/mo | Tiered plans |
| Postmark | 100/mo | $12.50/mo | ~$85/mo | Pay per email |
| Resend | 3,000/mo | $20/mo | ~$70/mo | Monthly + overage |
| Mailgun | Trial only | $15/mo | ~$75/mo | Tiered plans |
Prices are approximate based on publicly available pricing pages as of March 2026. Additional fees may apply for add-on features. Check each provider for current rates.
Provider overviews
RelayPost
A developer-first email platform that combines transactional email, campaign email, and SMTP relay in one service. Built by an independent team with transparent pricing and no venture-capital growth pressure. Best for teams that want one platform for all email needs.
SendGrid (Twilio)
The largest email API by market share, backed by Twilio. Offers the broadest feature set including email validation, marketing campaigns, and extensive integrations. Pricing is complex with add-on fees. Best for enterprises that need a full-featured platform and don't mind the cost.
Postmark
Owned by ActiveCampaign, Postmark has an excellent reputation for transactional email deliverability. They recently added a Bulk API for marketing/broadcast email. Pay-per-email pricing with tiered plans ($15-18/mo base for 10K). Best for teams that prioritize delivery speed and now want basic campaign support too.
Resend
A modern, developer-focused email API with a clean interface and React Email integration. Now supports SMTP relay and broadcast email via Audiences. Best for developers who want a well-designed API with growing feature coverage.
Mailgun (Sinch)
A mature email platform with strong API capabilities and email validation. Owned by Sinch. Pricing tiers are complex. No permanent free tier. Best for teams that need email validation built into their sending workflow.
Head-to-head comparisons
Can't decide between two specific providers? These detailed comparisons break down the differences:
- Resend vs Postmark — modern DX vs proven deliverability
- SendGrid vs Postmark — full-featured vs transactional specialist
- Migrate from SendGrid to RelayPost — step-by-step guide
- Migrate from Postmark to RelayPost — API mapping and migration checklist
How to choose the right email API
Consider these factors when evaluating email APIs:
- Do you need transactional email only, or campaigns too? RelayPost and SendGrid offer the most mature campaign support. Resend (Broadcasts) and Postmark (Bulk API) have added basic campaign features recently.
- Do you need SMTP relay? All five providers now support SMTP.
- What's your monthly volume? At 100K+ emails/mo, pricing differences become significant.
- How important is a free tier? Resend has the most generous free tier (3,000/mo). Mailgun has no permanent free tier.
- Do you need email validation? Only SendGrid and Mailgun offer it built-in.
Frequently asked questions
What is the best email API for developers?
It depends on your needs. For transactional + campaign email in one platform with simple pricing, RelayPost is a strong choice. For transactional-only with a proven track record, Postmark excels. For the largest ecosystem and enterprise support, SendGrid (Twilio) is the default. Resend offers a clean DX for transactional email.
Which email API has the best free tier?
Resend offers 3,000 emails/mo free (with a 100/day cap). RelayPost offers 1,000 emails/mo free with no daily cap. SendGrid offers 100 emails/day free. Postmark offers 100 emails/mo free. Mailgun offers a time-limited trial only.
Which email API is cheapest for 100K emails per month?
RelayPost Pro at $25/mo includes 100K emails. Resend would cost approximately $70/mo (Scale plan at $90/100K or Pro + overage). Mailgun Scale is approximately $75/mo. Postmark is approximately $85/mo at their per-email rate. SendGrid Pro is $89.95/mo. Prices are approximate — check each provider for current rates.
Do I need an email API or can I use SMTP?
Both work. An email API (REST) gives you programmatic control, webhooks, and easier integration with modern frameworks. SMTP works with any language or tool that supports it. All five providers now support both REST API and SMTP relay.
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