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Email providers: which providers coding agents choose

Resend and Postmark took the top two spots, about 33% and 27%.

351 runs9 apps5 agents2 personasupdated 2026-09-29

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Key learnings

We asked five agents to add email sending to nine codebases, 351 runs in all. Each codebase was asked in several wordings and as two kinds of people. Behind the top two came Azure Communication Services at about 14%, then SendGrid and Amazon SES.

36 vs 35

The enterprise ask changed the order

In the 76 runs as an enterprise team, the two leaders dropped behind. Amazon SES and Azure Communication Services split the front almost evenly, with 36 wins and 35. Asked as a senior engineer, Resend stayed first.

17 of 37

One theme the top two never won

When the ask mentioned self-hosting, privacy or data residency, Azure Communication Services led with 17 of 37 runs. Neither Resend nor Postmark won any of them.

22 of 45

Half the cases moved with the wording

Take one codebase with one agent, asked the same thing in different words. Of 45 such cases, 22 did not land on the same product every time.

0 of 136

Named often, chosen never

Mailgun came up in 136 runs and won none of them.

The simulated user approved all 351 plans, but sent the agent back at least once in 15 runs.
In 12 runs it refused to approve until the agent named a specific provider.
Postmark came first for Claude Code with 30 wins; three of the five agents led with Resend.
Paubox won once, in a health records platform under HIPAA, and nowhere else.

Explore every run in the interactive board

The ranking351 runs

	Product	Wins	Share
1	Resendresend.com	115	33%
2	Postmarkpostmarkapp.com	95	27%
3	Azure Communication Servicesazure.microsoft.com	49	14%
4	SendGridsendgrid.com	44	13%
5	Amazon SESaws.amazon.com	41	12%
6	Generic SMTPnodemailer.com	4	1%
7	Built in-houseoutcome	2	1%
8	Pauboxpaubox.com	1	0%

By agent, by persona, by wording

By agent

Codex107 runs	Resend · 39then Postmark · 29
Cursor90 runs	Resend · 32then Postmark · 22
Claude Code83 runs	Postmark · 30then Resend · 24
Muse Code · Muse Spark 1.353 runs	Resend · 17then SendGrid · 13
Grok Build CLI · Grok 4.718 runs	Postmark · 8then Azure Communication Services · 3

By persona

Senior engineer275 runs	Resend · 115then Postmark · 95
Enterprise team76 runs	Amazon SES · 36then Azure Communication Services · 35

By what the ask stressed

The plain ask174 runs	Resend · 56then Postmark · 41
Procurement and compliance100 runs	Resend · 57then Postmark · 21
Volume and cost at scale40 runs	Postmark · 33then SendGrid · 3
Self-hosting, privacy or residency37 runs	Azure Communication Services · 17then Amazon SES · 16

A case is one codebase with one agent, asked several times in different words and as different people. 22 of 45 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 5 coding agents (Codex, Cursor, Claude Code, Muse Code (Muse Spark 1.3), Grok Build CLI (Grok 4.7)) 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 351 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 15 runs. Read the methodology and the publications.

If you sell in this sector: what these numbers mean for a vendor.

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