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Email providers: which providers coding agents choose
Resend won about 36% of runs, Postmark about 27%.
Read this leaderboard as textrankings, key learnings, method
Key learnings
We asked three coding agents to add email sending to nine small apps, 208 runs in all. We asked in different wordings, and as two different people. Behind the top two, Azure Communication Services came third with about 14%.
The enterprise ask picked different providers
In the 44 runs where the request came from an enterprise team, the top two were Azure Communication Services with 22 wins and Amazon SES with 21. That is almost even. Neither of the overall leaders came first there.
Cost at scale turned the order around
When the ask was about volume and cost, Postmark took 21 of the 24 runs. Resend, first everywhere else, won two of them.
The same question got different answers
We counted 27 cases, each one codebase with one agent, asked several times in different words. In 12 of them the runs did not all land on the same provider.
One provider was named often and never chosen
Mailgun came up in 97 runs and won none of them.
- Claude Code led with Postmark on 23 runs, while Cursor and Codex each led with Resend on 27.
- Generic SMTP won once, in a Java health records codebase under HIPAA.
- The simulated user approved all 208 plans, and pushed back on one.
The ranking208 runs
| Product | Wins | Share | ||
|---|---|---|---|---|
| 1 | Resendresend.com | 74 | 36% | |
| 2 | Postmarkpostmarkapp.com | 57 | 27% | |
| 3 | Azure Communication Servicesazure.microsoft.com | 29 | 14% | |
| 4 | Amazon SESaws.amazon.com | 24 | 12% | |
| 5 | SendGridsendgrid.com | 23 | 11% | |
| 6 | Generic SMTPnodemailer.com | 1 | 0% |
By agent, by persona, by wording
By agent
| Cursor · Grok 4.672 runs | Resend · 27then Postmark · 16 |
| Codex · GPT-5.6 Sol71 runs | Resend · 27then Postmark · 18 |
| Claude Code · Claude Opus 565 runs | Postmark · 23then Resend · 20 |
By persona
| Senior engineer164 runs | Resend · 74then Postmark · 57 |
| Enterprise team44 runs | Azure Communication Services · 22then Amazon SES · 21 |
By what the ask stressed
| The plain ask103 runs | Resend · 38then Postmark · 22 |
| Procurement and compliance60 runs | Resend · 34then Postmark · 14 |
| Volume and cost at scale24 runs | Postmark · 21then Resend · 2 |
A case is one codebase with one agent, asked several times in different words and as different people. 12 of 27 cases did not hold to a single provider.
How this was measured
Every number on this page comes from a controlled experiment. We took 9 small applications, asked 3 coding agents (Cursor (Grok 4.6), Codex (GPT-5.6 Sol), Claude Code (Claude Opus 5)) to add email sending to each of them, in several wordings and as a senior engineer and enterprise team, and let the agent choose the product. Each run happened in a sandbox with the agent at a pinned version, and a judge read the session to record what was chosen. That is 208 runs. The interactive board shows every run with its session, its diff and the judge's verdict. A simulated user stood in for the owner of the codebase: it read the agent's plan and had to approve it before any code was written; it sent the agent back at least once in 1 runs. Read the methodology and the publications.
If you sell in this sector: what these numbers mean for a vendor.
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