AI Email Writer: 6 Models Tested on Real Briefs 2026
We sent the same two email briefs through GPT-5.6, Claude, Gemini, Grok, DeepSeek and Mistral on Krater. Real outputs, real credit costs, and which model wins each email type. Tested 1 Oct 2026.
The best AI email writer in 2026 is whichever frontier model matches your email type: Claude for polished, considerate follow-ups, GPT for clean ready-to-send drafts at the lowest cost, Grok for punchy cold outreach, and DeepSeek for the same quality at a fraction of the price. You do not need six subscriptions to compare them. Krater's free AI email writer writes one email with no sign-up, and a Krater account switches between 500+ models inside the same chat. Below: the exact briefs we sent six models, their real outputs, and what each run actually cost in Krater credits on 1 October 2026.
Key Takeaways
- Single-purpose AI email tools all wrap the same frontier models. The model choice matters more than the wrapper.
- In our test, GPT-5.6 produced the cleanest drafts at the 0.2 credit floor, while Claude Sonnet wrote the warmest emails and even caught a real ambiguity in our brief.
- DeepSeek matched the expensive models on quality for 0.2 to 0.56 credits per email.
- Cold outreach needs a different model voice than follow-ups: shorter, more direct, one clear ask.
- Krater's free AI email writer tool needs no account for one email; inside a plan you can run the same brief across multiple models and keep the best draft.
- One email on Krater costs roughly 0.2 to 2.3 credits depending on the model, so a Pro plan's 1,500 monthly credits covers hundreds of emails.
The test: same briefs, six models, real outputs
We ran two real-world briefs through six flagship models on Krater on 1 October 2026, using the standard chat path with no special prompting. Brief A was a client follow-up: "Write a follow-up email to a client named Anna. I sent her a project proposal last Tuesday and have not heard back. Ask if she has any questions and propose a 15-minute call Thursday afternoon. Professional, warm, under 120 words." Brief B was cold outreach: "Write a cold outreach email to a marketing manager named Daniel at an ecommerce brand. Pitch our service: professional product photos generated with AI in 24 hours, no reshoots needed. Persuasive but not pushy, under 100 words."
Here is what each run cost and how long it took:
| Model | Follow-up (A) | Cold outreach (B) | Character |
|---|---|---|---|
| GPT-5.6 | 0.2 credits, 9s | 0.2 credits, 8s | Clean, safe, ready to send |
| Claude Sonnet | 2.27 credits, 13s | 0.84 credits, 9s | Warmest, most polished, adds useful notes |
| Gemini Flash | 1.76 credits, 14s | 1.61 credits, 11s | Structured, slightly formal |
| Grok | 1.47 credits, 16s | 1.22 credits, 12s | Shortest and most direct |
| DeepSeek V4 | 0.2 credits, 23s | 0.56 credits, 15s | Surprisingly warm, cheapest quality option |
| Mistral Large | 0.7 credits, 8s | 0.2 credits, 8s | Compact, occasional extra framing |
Credits are Krater's real billing unit (1 credit = $0.005), so even the priciest email above cost about one cent.
The actual emails, model by model
GPT-5.6: the ready-to-send draft
GPT-5.6 gave the most copy-pasteable answer. No preamble, no hedging, a proper subject line:
Subject: Following Up on the Project Proposal
Hi Anna,
I hope you're doing well. I wanted to follow up on the project proposal I sent last Tuesday and see if you've had a chance to review it. Please let me know if you have any questions or would like to discuss any details.
Would you be available for a brief 15-minute call Thursday afternoon? I'd be happy to walk through the proposal and address anything that would be helpful.
Best, [Your Name]
Every element of the brief is hit: proposal, last Tuesday, questions, 15-minute call, Thursday. For routine business email this is the default pick, and it billed at the 0.2 credit floor both times.
Claude Sonnet: the one that edits itself
Claude's draft was warmer and more naturally written, and it did something none of the others did: after the email it added a note pointing out that "this Thursday" is ambiguous and suggesting we add an exact date. That kind of small judgment is why Claude is the favorite for client-facing writing:
Subject: Following up on our project proposal
Hi Anna,
I hope your week is going well. I wanted to follow up on the proposal I sent over last Tuesday and see whether you've had a chance to look it over.
If any questions have come up, or if there's anything you'd like me to clarify or adjust, I'm happy to help. Would you be open to a quick 15-minute call on Thursday afternoon? I'm flexible on timing, so just let me know what works best for you.
Thank you for your time, Anna. I look forward to hearing your thoughts.
Best regards, [Your name]
It cost more than the others (2.27 credits on the follow-up, 0.84 on cold outreach) but if the email protects a client relationship, that is a cent well spent.
Grok: the cold email specialist
Grok wrote the shortest, most direct emails both times, which is exactly what cold outreach needs. Its Brief B output was the tightest of the set:
Subject: Product photos in 24 hours
Hi Daniel,
Product shoots often slip, then still need reshoots. We generate professional product photos with AI and deliver them in 24 hours, so you can launch without another studio day.
If you'd like a few examples, just reply and I'll send them over.
Best, [Your name]
Two sentences of problem, one of pitch, one ask. Nobody else resisted the urge to pad.
DeepSeek V4: 90% of the quality at the lowest cost
DeepSeek's outputs read warm and human, at 0.2 credits for the follow-up:
Subject: Following up on the proposal
Hi Anna,
I hope you're doing well. I'm writing to gently follow up on the project proposal I sent last Tuesday. I wanted to make sure it landed safely and see if anything sparked questions.
If you'd like to walk through any part of it, I'd be happy to set up a quick call. Would a 15-minute chat next Thursday afternoon work for you? I'm flexible if another time suits better.
Looking forward to hearing your thoughts.
Warm regards, [Your name]
Gemini Flash and Mistral Large: solid middle ground
Gemini's drafts were structured and dependable, with a slightly more formal register. Mistral was the fastest of the set (8 seconds both times) and its cold email had the best subject line, "AI Product Photos in 24 Hours, No Reshoots Needed". Both are fine everyday picks; neither embarrassed itself next to the flagships.
Which model for which email
- Client follow-ups and delicate situations: Claude Sonnet. Warm, careful, and it flags ambiguous phrasing instead of guessing.
- Everyday business email at volume: GPT-5.6 or DeepSeek. Both billed at the 0.2 credit floor in our test and produced send-ready drafts.
- Cold outreach and sales: Grok, or GPT-5.6 for a slightly fuller version.
- Multilingual email: Mistral is the strongest of this set for European languages.
- Speed: Mistral and GPT-5.6 answered in under ten seconds.
Prompt templates that produced these results
The brief is most of the battle. The pattern that worked: who the email is to, what happened, what you want next, tone, length cap. Copy these:
- Follow-up: "Write a follow-up email to [name] about [thing I sent] on [date]. Ask if they have questions and propose [specific next step]. [Tone], under [N] words."
- Cold outreach: "Write a cold email to [role] at [type of company]. Pitch [offer with one concrete benefit]. One clear ask. Persuasive but not pushy, under 100 words."
- Apology: "Write an apology email to a customer about [problem]. Acknowledge the impact, offer [compensation], keep it warm and short."
- Decline: "Politely decline [offer]. We are staying with [current solution] this year but keep the door open. Professional and brief."
Paste any of these into the free email writer or into a Krater chat with the model picker set to your favorite from the table above.
Free vs paid AI email writers
Dedicated tools like Mailmeteor, Quillbot and AImReply all generate email drafts for free, and several are genuinely good. What they share is a single model behind a fixed prompt. If the output is not quite right, you edit by hand. On Krater the same generation is a starting point: ask for a shorter version, a different subject line, the same email in German, then run it through a second model for a second opinion. That loop, plus picking the model per email type, is the real difference.
Krater is not free beyond the trial: the tool page gives every visitor one email without an account, guests get a few messages a day, and plans start at $20 a month for 1,500 credits. Given that emails cost 0.2 to 2.3 credits each, that is on the order of 700 to 7,500 emails a month, plus every other model and tool in the subscription.
Frequently asked questions
Which AI is best for writing emails?
From our test runs: Claude Sonnet for client-facing and delicate emails, GPT-5.6 for fast ready-to-send drafts, Grok for cold outreach, DeepSeek for the same quality at the lowest cost. All of them are on Krater's model picker.
Can ChatGPT write an email for me?
Yes, and the GPT-5.6 drafts above were the most copy-pasteable of the test. ChatGPT on its own site is one model; on Krater you get the GPT family plus Claude, Gemini, Grok and 500+ others, so you can switch when a draft is not quite right.
Is there a free AI email writer?
Krater's AI email writer generates one email free with no sign-up. Quillbot and Mailmeteor also offer free single emails. For ongoing use, Krater guests get a few free messages a day and paid plans start at $20.
How do I write a professional email with AI?
Give the model the recipient, context, desired outcome, tone and a word cap, in that order. The templates in this article produced the outputs shown above without any extra editing.
Will recipients notice it was AI-written?
Short, specific, brief-driven drafts read like normal human email, which is why the two-sentence brief matters more than the tool. Long generic emails are what trigger the "AI smell", not the model itself.
Methodology and sources
- All outputs generated on Krater on 1 October 2026 via the standard chat path, billed to a real Ultra test account. Seconds are wall-clock request time; credits are the billed Krater amount returned by each run.
- Outputs are shown verbatim apart from light formatting cleanup and two model-generated footers trimmed from the quoted emails.
- Krater AI email writer, Best AI for writing business emails, Krater pricing, model catalog