# Transactional email: which vendors AI models recommend (September 2026)

Source: Memetik Index, https://www.memetik.ai/index/transactional-email
Field: Startup stack. Edition: 2026-09. Run: 2026-09-02.
Method: 5 fixed buyer prompts sent once each to 10 current models (GPT-5.6 Sol, GPT-5.6 Terra, GPT-5.6 Luna, Claude Opus 5, Claude Sonnet 5, Claude Fable 5, Gemini 3.6 Flash, Gemini 3.5 Flash, Sonar Pro, Sonar Reasoning Pro) via their APIs with web search enabled. 50 answers. Answer share = fraction of answers naming the vendor. Named first = fraction where it was the first tracked vendor named.

**Resend leads** with 100% answer share (named in 50 of 50 answers) and was named first in 56%. Amazon SES follows at 96%. 12 vendors were named at least once.

## Answer share

| # | Vendor | Answer share | Named first | Mentions | Models |
|---|---|---|---|---|---|
| 1 | Resend | 100% | 56% | 50/50 | GPT-5.6 Sol, ChatGPT, GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 2 | Amazon SES | 96% | 4% | 48/50 | GPT-5.6 Sol, ChatGPT, GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 3 | Postmark | 94% | 30% | 47/50 | GPT-5.6 Sol, ChatGPT, GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 4 | SendGrid | 86% | 2% | 43/50 | GPT-5.6 Sol, ChatGPT, GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 5 | Mailgun | 78% | 0% | 39/50 | GPT-5.6 Sol, ChatGPT, GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 6 | Mailtrap | 52% | 8% | 26/50 | Claude Opus 5, Claude, Claude Fable 5, Gemini, Gemini 3.5 Flash, Perplexity, Sonar Reasoning Pro |
| 7 | Brevo | 26% | 0% | 13/50 | GPT-5.6 Luna, Claude Opus 5, Claude, Claude Fable 5, Gemini, Perplexity, Sonar Reasoning Pro |
| 8 | Loops | 10% | 0% | 5/50 | Claude Opus 5, Gemini, Gemini 3.5 Flash |
| 9 | Knock | 10% | 0% | 5/50 | Claude Opus 5, Claude, Claude Fable 5 |
| 10 | SparkPost | 4% | 0% | 2/50 | GPT-5.6 Sol, GPT-5.6 Luna |
| 11 | Mailjet | 2% | 0% | 1/50 | Claude |
| 12 | Plunk | 2% | 0% | 1/50 | Claude Opus 5 |

## By model

- **GPT-5.6 Sol** (OpenAI, gpt-5.6-sol, 5 answers): Resend 100%, Amazon SES 100%, Postmark 80%, SendGrid 80%, Mailgun 60%
- **GPT-5.6 Terra** (OpenAI, gpt-5.6-terra, 5 answers): Resend 100%, Amazon SES 100%, SendGrid 100%, Postmark 60%, Mailgun 60%
- **GPT-5.6 Luna** (OpenAI, gpt-5.6-luna, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, SendGrid 100%, Mailgun 100%
- **Claude Opus 5** (Anthropic, claude-opus-5, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, Mailgun 100%, SendGrid 80%
- **Claude Sonnet 5** (Anthropic, claude-sonnet-5, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, Mailgun 100%, SendGrid 80%
- **Claude Fable 5** (Anthropic, claude-fable-5, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, SendGrid 100%, Mailgun 80%
- **Gemini 3.6 Flash** (Google, gemini-3.6-flash, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, SendGrid 80%, Mailtrap 80%
- **Gemini 3.5 Flash** (Google, gemini-3.5-flash, 5 answers): Resend 100%, Amazon SES 100%, Postmark 100%, Mailgun 80%, Mailtrap 80%
- **Sonar Pro** (Perplexity, sonar-pro, 5 answers): Resend 100%, Postmark 100%, SendGrid 100%, Amazon SES 80%, Mailgun 60%
- **Sonar Reasoning Pro** (Perplexity, sonar-reasoning-pro, 5 answers): Resend 100%, Postmark 100%, SendGrid 100%, Mailtrap 100%, Amazon SES 80%

## Sources the models cited

- mailtrap.io: 193
- nitrosend.com: 112
- agentmail.to: 112
- smtp2go.com: 104
- sequenzy.com: 85
- reddit.com: 58
- maestra.io: 37
- ventureharbour.com: 37
- mailtoolscout.com: 26
- emailvendorselection.com: 24
- postmarkapp.com: 22
- sendlayer.com: 21
- loops.so: 16
- mailgun.com: 15
- bavimail.com: 15

## Prompts

1. What is the best transactional email API for a developer? Name specific products.
2. Which transactional email API would you recommend to a developer in 2026?
3. Compare the top transactional email API options right now.
4. I'm a developer and I need a transactional email API. What should I use and why?
5. Best transactional email API for a SaaS sending signup and receipt emails?

## Tracked but never named

Customer.io, Courier

Raw answers: https://www.memetik.ai/index/transactional-email (every answer published in full). Chart image: https://www.memetik.ai/charts/transactional-email.png. Licence: CC BY 4.0, credit "Memetik Index".