If you build apps with an AI assistant, sooner or later it has to send a signup or password reset email, and you ask it which service to use. We asked ChatGPT, Claude, Gemini, Perplexity and Grok 4 versions of that question, 3 times each: 60 answers. One provider won almost everywhere. The source the engines leaned on most was a company that wasn’t even on our list.
Three Findings, Up Front #
- Resend is the default. It was named in 55 of 60 answers (92%), and in every single answer from 4 of the 5 engines.
- ChatGPT is the exception. It named Resend in 7 of 12 answers, behind Mailgun (10) and Postmark (9). If you ask ChatGPT, you get a different default.
- A vendor blog wrote the answers. The most cited site was sequenzy.com, in 25 of 60 answers, ahead of resend.com itself (18). Sequenzy has a comparison post for nearly every question we asked, and the engines named it in 6 answers.
ChatGPT Goes Its Own Way #
Claude, Gemini, Perplexity and Grok named Resend in every answer. ChatGPT spreads its picks across the older providers, and reaches for Postmark when the job is signup and password reset email:
| Engine | Most named | Runner-up |
|---|---|---|
| **Grok** grok-4.3 | Resend (12/12) | Postmark (12/12) |
| **ChatGPT** gpt-5.4-mini | Mailgun (10/12) | Postmark (9/12) |
| **Claude** claude-haiku-4-5 | Resend (12/12) | Mailtrap (9/12) |
| **Perplexity** sonar | Resend (12/12) | Postmark (9/12) |
| **Gemini** gemini-3.5-flash | Resend (12/12) | Amazon SES (9/12) |
The other engines are close to unanimous. Gemini, asked which provider works best for apps built with Lovable or Bolt, put it this way:
The Vendor Blog Behind the Answers #
Grounded engines search the web before they answer, then cite what they read. For email APIs, the page they read most belongs to Sequenzy, an email platform. Its blog has a post for almost every question in this test: best transactional email services, email tools for Lovable, for bolt.new, for Supabase, sending email from Next.js.
| Site the engines cited | Answers citing it | Times cited |
|---|---|---|
| sequenzy.com | 25/60 | 47 |
| resend.com | 18/60 | 22 |
| docs.lovable.dev | 9/60 | 19 |
| mailtrap.io | 13/60 | 19 |
| brevo.com | 17/60 | 17 |
| elasticemail.com | 10/60 | 17 |
| makerkit.dev | 5/60 | 17 |
| courier.com | 12/60 | 15 |
| lovable.dev | 9/60 | 15 |
| support.bolt.new | 9/60 | 14 |
sequenzy.com was cited 47 times across 25 answers, about 30 of those citations from Perplexity. It pays off. Sequenzy was not one of the providers we tracked, yet the engines recommended it by name in 6 answers:
The platforms themselves shape the answer too. Lovable’s docs were cited in 9 answers. If a builder platform documents one email provider, that provider inherits the platform’s authority in AI answers.
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What This Means If You Sell to Builders #
- Write the page for the exact question. Sequenzy didn’t write one “best email API” post. It wrote one per platform and stack, and each one matches a question builders actually ask.
- Get into the platform docs. Lovable’s and Bolt’s own docs and support pages are in the cited list above. An integration guide there is worth more than another page on your own site.
- Check ChatGPT on its own. A blended number hides that the biggest engine disagrees with the other four. Measure per engine.
Disclosure: openllmrank sends its own email through Postmark, and sells the kind of AI-visibility measurement used in this post.
Frequently Asked Questions #
Which email API do AI engines recommend for sending app emails?
Resend. It was named in 55 of 60 answers (92%) across ChatGPT, Claude, Gemini, Perplexity and Grok, ahead of Postmark (75%), Amazon SES (70%) and SendGrid (60%).
Does ChatGPT recommend Resend?
Less than the other engines do. ChatGPT named Resend in 7 of its 12 answers, and named Mailgun (10) and Postmark (9) more often. The other four engines named Resend in all 12 of theirs.
Where do AI engines get email API recommendations from?
Mostly from comparison posts on vendors' own blogs, plus the docs of the platforms people build on. The most cited site was sequenzy.com, cited in 25 of 60 answers, more than resend.com (18).